COMPRESSED AIR 
PRODUCTION 



OR THE 



THEORY AND PRACTICE 



OF 



AIR COMPRESSION, 



BY 






W. L. SAUNDERS. 



Published by 

COMPRESSED AIR, 

26 Cortlandt Street, 

New York, 









18436 



Copyrighted 1898, 
By W. I,. Saunders 




WD COPIES RECEIVED. 



°\ 



. 



2nd COPY, 
1898. 



ko o^K^cAVAS 



COMPRESSED AIR PRODUCTION, 



By W. L. SAUNDERS. 

Compressed air is air under pressure. It is usual to define com- 
pressed air as air increased in density by pressure, but we may produce 
compressed air by heat alone, as illustrated by the discharge of a cork 
from an empty bottle when heated. Though one of the oldest of the 
sciences, compressed air is, in its development and use, one of the young- 
est. Hero, of Alexandria, a century before Christ experimented and 
wrote upon "Pneumatics," calling special attention to the influence of heat 
in expanding and contracting air. It is said that Hero put into prac- 
tical use an invention by which the opening and closing of temple doors 
was effected by the alternate rarefaction and condensation of air which 
was brought in contact with heated and cooled surfaces of altar tops. 
Yet the science of pneumatics played no important part in industrial 
progress until scarcely more than a century ago it came into general use 
for diving-bells, and was later on applied by Brunei to caisson work. 

In 1830 the French Academy of Sciences gave a medal to Thillorier 
for his method of compressing gases by stages. . In 1849 the Baron 
von Rathen suggested the use of compressed air at 750 pounds pressure 
per square inch in locomotives-; It is a singular fact that the Baron, in 
describing the method by which he proposed to attain this high pressure, 
advised compound compressors with inter-coolers. The special advan- 
tages of cooling the air between the different stages of compression were 
set forth. This stage compression and inter-cooling is one of the most 
important recent improvements made in air-compressing machinery. 

Until recent years the use of compressed air in America has been 
confined almost exclusively to mining, tunneling, bridge building, or to 
work in a confined space for which no other power was available. Elec- 
tricity has recently become a competitor of compressed air in that it, too, 
may be used in confined spaces, and may be transmitted long distances 
and distributed. Until such competition arose the question of producing 
compressed air economically was but little agitated. The attention of 
engineers was mainly devoted to the development of an apparatus for 
using compressed air, it being taken for granted that air was an expensive 
power at best. The manufacturer sought to perfect his compressor on 
lines of low first cost, light weight, economy of space and p-eneral avail- 
ability. Dry, pure air, delivered at a sufficient pressure by a machine 
which could be depended upon, has been the controlling consideration. 

♦Reprinted from Compressed Air. 



4 COMPRESSED AIR PRODUCTION. 

Compressed air and air-compressing machinery have been consid- 
ered, and the science developed by two classes of men — the practical men 
and the engineers. The practical men confined their work to the ma- 
chine. The confusing diagrams and figures of the engineers were not 
considered, because they were not understood. The engineers took occa- 
sional plunges into compressed air theories, producing figures controvert- 
ing certain well-established and so-called practical facts, and almost in- 
variably basing the conditions of compressed air economy upon questions 
of thermo-dynamics. The problems produced by the engineers were too 
mathematical for the practical compressed air men. These men knew too 
little of the theory of compressed air, hence progress in the science has 
been slow. 

During recent years an impetus nas been given to compressed air 
development by the strides made by electricity, and by the increased use 
of compressed air in the arts. Electricity, although apparently a com- 
petitor, has really played the part of a friend in pointing out the possi- 
bilities of transmission and use of air in directions before unknown; thus 
a market has been created. 

The perfection of the air compressor on lines of economy naturally 
followed the wide use of compressed air in competition with steam and 
electricity. The best steam engine practice has been applied to the com- 
pressor. Compound condensing Corliss engines are now used in con- 
nection with air cylinders of new design. 

The whole subject of compressed air may be divided into three heads: 
Production. 
Transmission. 
Use. 

No better evidence is needed of the obscurity of the science, even 
among engineers, than the fact that it is the usual thing to look upon 
compressed air as an expensive power, because of the great loss which is 
suffered during transmission. The great losses and the serious diffi- 
culties encountered in reality do not belong to transmission. Com- 
pressed air power may be transmitted and distributed with no greater 
difficulty than the distribution and transmission of illuminating gas. It 
is a question of the size of pipe, volume and the pressure. There is not a 
properly designed compressed air installation in operation to-day that 
loses over five per cent, by the transmission alone. The question is alto- 
gether one of the size of pipe, and if the pipe is large enough the friction 
loss is a small item. It is undoubtedly true that there are places where a 
conduit has been laid for a certain volume of air, and where the supply 
has been increased without increasing the size of the conduit, the result 
of this being that more air is forced through the pipe than its sectional 
diameter will admit economically; hence the velocity of flow is increased, 
and as the friction is in direct proportion to the velocity the loss of power 
is also increased. The largest compressed air long-distance power plant 
in America is that at the Chapin Mines in Michigan, where the power 
is generated at Quinnesec Falls, and transmitted three miles. This is not 
an economical plant, but the loss of pressure, as shown by the gauge, 



COMPRESSED AIR PRODUCTION. 5 

is only two pounds, and this is the loss which may be laid strictly to trans- 
mission. During the construction of the Jeddo Tunnel, near Hazelton, 
Pa., compressed air at 60 pounds pressure was conveyed 10,860 feet from 
the central station. The writer was called upon to explain a mysterious 
condition which existed on both ends of the line. The pressure gauge 
recorded the same figures and the gauges were sent to the shops for re- 
pairs, because everybody was convinced that " something was wrong." 
The result was not changed when the gauges had been " repaired/' it 
being evident that this apparently perfect economy of transmission was 
due to the fact that a large pipe (nearly six inches in diameter) was used at 
that time to convey so small a volume of air that the velocity in the pipe 
produced so small a friction loss that it could not be recorded on the 
gauge. 

Having defined compressed air, we must next define heat, for in 
dealing with compressed air we are brought face to face with the com- 
plex laws of thermo-dynamics. When we produce compressed air we 
produce heat, and when we use compressed air as a power we produce 
cold. Based on the material theory of heat, it was said that when we take 
a certain volume of free air and compress it into a smaller space we gtt 
an increase of temperature, because we have the heat of the original vol- 
ume occupying less space; but no one at this date accepts the material 
theory of heat. The science of thermo-dynamics teaches that heat and 
mechanical energy are only different phases of the same thing, the one 
being the motion of molecules and the other that of masses. This is the 
accepted theory of heat. In other words, we do not believe that there 
is any such thing as heat, but that what we call heat is only the sensible 
effect of motion. In the cylinder of an air compressor the energy of the 
piston is converted in molecular motion in the air, and the result or the 
equivalent is heat. A higher temperature means an increased speed of 
vibration, and the lower temperature means that this speed of vibration is 
reduced. If we hold an open cylinder in one hand and a piston in the 
other, and place the piston within the cylinder, we here have a confined 
volume of air at normal temperature and pressure. These particles of 
air are in motion and produce heat and pressure in proportion to that 
motion. Now, if we press the piston to a point in the center of the cylin- 
der, that is, to one-half the stroke, we here decrease the distance between 
the cylinder head and the piston just one-half; hence each molecule of 
air strikes twice- as many blows upon the piston and head in traveling the 
same distance, and the pressure is doubled. We have also produced heat 
(about 116 degrees), because we have expended a certain amount of work 
upon the air; the air has done no work in return, but we have increased 
the energy of molecular vibration in the air, and the result is heat. 

But what of this heat? W r hat harm does it do? If we instantly re- 
lease the piston which we have forced to one-half stroke, it will return 
to its original position less only a fractional part, due to friction. We 
have, therefore, recovered all or nearly all the power spent in compressing 
the air. We have simply pressed and released a spring, and this illus- 
tration shows what a perfect spring compressed air is. We see also the 



COMPRESSED AIR PRODUCTION. 



possibility of expending one horse-power of energy upon air and getting 
almost one horse-power in return. Such would be the case in practical 
work if we could use the compressed air power immediately and at the 
point where the compression took place. This is scarcely possible, as the 
heat in the air is soon lost by radiation, and we have lost power. 

Thirteen cubic feet of free air at normal temperature and barometric 
pressure weigh about I pound. In the illustration referred to about 116 
degrees of heat are liberated at half stroke of a compressor. The gauge 
pressure at this point reaches 24 pounds. According to Mariotte's law, 
'*the temperature remaining constant, the volume varies inversely as the 
pressure," we should have 1 $ pounds gauge pressure at half stroke. The 
difference, 9 pounds, represents the effect of the heat of compression in 
increasing the relative volume of the air. 

The specific heat of air under constant pressure being 0.238, we 
have 0.238x116=27.6 heat units produced by compressing one pound, or 
13 cubic feet, of free air into one-ialf its volume; 27.6x7/ '2 (Joule's equiva- 
lent) 3 ^ 1,3 07 foot pounds. We know that 33,000 foot pounds is one 
horse-power, and we see how easily about two-thirds of a horse-power in 
heat units may be produced and lost in compressing one pound of air. 
Exactly this same loss is suffered when compressed air does work in an 
engine without reheating, and is expanded down to its original pressure. 
In other words, the heat of compression and the cold of expansion are in 
degree equal. 

Figure 1 is a sketch designed to indicate graphically the effect of heat 
and cold in compression and expansion of air. 

Rt«t '^jijUx.YJJ ft 1 ■& aYh. « t.w\ *\ ExpATV*T,oivii<$ Conltviwiouft H«eftvng. ? 




The sketch illustrates an open cylinder, which may serve both as an air 
compressor and an air engine. The piston at the point shown is sup- 
posed to confine a volume of free air in the cylinder and at a temperature 
of 60 degrees; let it be pressed down until it reaches the point indicated 



COMPRESSED AIR" PRODUCTION. 7 

by 45 pounds, and the pressure will follow the dotted lines marked "Adia- 
batic." This is, of course, assuming that the heat, which is invariably 
produced by compression, is suffered to remain in the air and to in- 
fluence the pressure. We here have a confined volume of compressed air 
at a pressure of 45 pounds and a temperature of 320 degrees. Let there 
be no absorption of heat and the piston if released will return to the 
starting point, the pressure following exactly the line indicated during 
compression and the temperature returning to 60 degrees. In such a 
case we assume, of course, that the piston is frictionless. This points to 
the fact that compressed air is a perfect spring, and that the heat of com- 
pression when utilized can be made to return its full value of energy. 

An Air Compressor provided with no cooling device would show a 
pressure line following closely that marked "Adiabatic" on Fig. 1. The 
hot compressed air confined in the cylinder at the 45-pound point, if 
transferred through pipes to an air engine and maintained hot until 
used, would be available for work in the same proportion (less a little 
friction) as we have shown in the theoretical case where the piston, used 
as a compressor, is driven back to the starting point. Practically, it is 
impossible to convey hot compressed air any distance from the com- 
pressor, for air, though very slow in taking up heat, has so low a specific 
heat that it parts with its temperature rapidly. Steam having a higher 
specific heat may be conveyed as a power even through naked pipes, and 
this fact has led to mistakes in regard to the possibilities with compressed 
air. 

Returning to Fig. 1, let us imagine that the piston has been stopped 
at the 45-pound point, and that the compressed air, which, as we have 
seen, has a temperature of 320 degrees, is transferred into a receiver and 
used at a point say half a mile distant. The temperature will now be re- 
duced to that of the surrounding medium, or to the initial temperature, 
which the sketch shows to be 60 degrees; and if the system is well de- 
signed, that is, if the pipes are large enough and there are no leaks or 
other irregularities, we will have nearly 45 pounds oressure on the other 
end of the line, as there is a direct elastic medium between the two points. 
But the volume will be reduced in size, because of the reduced tempera- 
ture, and will correspond with the space underneath the lowest dotted line 
in the figure marked "Volume I." If it is now used without reheating 
to do work in an engine, the line of reduction in pressure will follow the 
lower dotted line marked "Adiabatic" until it reaches the point marked 
201 degrees, which represents the theoretical temperature of the air when 
exhausted at atmospheric pressure. We now see that the piston, instead 
of returning to the starting point, has only had power enough behind it to 
return it to a point about half way. 

The illustration points to the importance in compressed air economy 
of reducing to the lowest point practicable the temperature of the air 
before and during compression and conversely increasing to the highest 
point the temperature before and during use or expansion. 

If, in the case referred to, heat had been applied during expansion, 



8 



COMPRESSED AIR PRODUCTION. 



the pressure would follow the line marked "Isothermal Expansion, 
the piston might be returned to the starting point. 

Tabi,e i. — Of the volume and weight of dry air 
at different temperatures under a constant 
atmospheric pressure of 29.92 inches of mer- 
cury in the barometer (one atmosphere), 
the volume at 32 ° fahrenheit being i. 



and 







J3 






£ 






£ 


V 

5 a 


■1-1 D 


<4M 3 


5 .- 


fl-J 


"I 


V 
u 

•2 «5 


.2tJ 




fl) C U 




4J fl 1- 

p,- be 


4) 4; 


U 






11 


IS v 

1 * 


1! 


3 


s 3 


n 


H 


£ 3 

J" 1 


1= 

> 


1-s 















00540 


1,300 


3595 





32 


1-000 


0-0807 


275 


1-495 


00225 


42 


1-020 


0-0791 


300 


1-546 


0522 


1,400 


3-789 


0-0213 


52 


1-041 


00776 


325 


1-597 


0506 


,1,500 


3993 


0-0202 


62 


1-061 


0761 


350 


1-648 


0490 : 


1,600 


4197 


0192 


72 


1-082 


00747 


375 


1-689 


0-0477 


1,700 


4-401 


0-0183 


83 


' 1-102 


00733 


400 


1-750 


0461 


1,800 


4-605 


0-0175 


92 


1122 


0-0720 


450 


1-852 


00436 


1,900 


4-809 


0168 


102 


1*143 


0-0707 


500 


1954 


0413 


2,000 


5 012 


00161 


112 


1163 


00694 


550 


2 056 


00381 


2,100 


5-216 


00155 


122 


1-184 


0682 


600 


2-158 


00376 


2,200 


5-420 


00149 


132 


1-201 


00671 


650 


2-260 


0-0357 


2,300 


5624 


0-0142 


142 


1-224 


0-0660 


700 


2 362 


0-0338 


2,400 


5-828 


0-0138 


152 


1-245 


0-0649 


750 


2-464 


0-0328 


2,500 


6-032 


0-0133 


162 


1-255 


0638 


800 


2566 


0315 


2:600 


6-236 


0-0130 


]?2 


1-285 


0-0628 


850 


2-668 


00303 


2,700 


6440 


0-0125 


16*5 


1-306 


0-0618 


900 


2-770 


00292 


2,800 


6 644 


00121 


192 


1326 


00609 


950 


2-872 


0-0281 


2,900 


6847 


0-0118 


202 


1-347 


0-0600 


1,000 


2974 


0268 


3 V 000 


7051 


00114 


212 


1-367 


00591 


1,100 


3-177 


00254 


3,100 


7255 


o-oiii 


230 


1-404 


00575 


1,200 


3381 


0-0239 


3,200 


7-459 


0-0108 


250 


1-444 


00559 















Another case is shown by the sketch in which the air is compressed 
adiabatically to 45 pounds pressure and heat enough is applied during ex- 
pansion to maintain the temperature at 320 degrees until the air is ex- 
hausted at atmospheric pressure. This case is purely theoretical and 
illustrates the possibility of obtaining more power out of a given volume 
of air after compression than was expended at the compressor. 

Experiments made by M. Regnault and others on the influence of heat 
on pressures and volumes of gases have enabled us to fix the absolute 
zero of temperature as about — 461 degrees Fahrenheit. This point, — 461 
degrees below zero, has been taken to be the theoretical point at which 
a volume of air is reduced to nothing. The exact figures representing 
absolute zero vary with different authors, but for all practical purposes 
— 461 degrees F. is near enough. The volume of air at different tempera- 
tures is in proportion to the absolute temperature, and on this basis Box 
has produced table 1. 

The effect of the heat of compression in increasing the volume, and 
the heat produced at different stages of compression, are shown by table 
2 (Box) : 

A cubic foot of free air at a pressure of one atmosphere (equal to 14.7 
pounds above a vacuum) at a temperature of 60 degrees when compressed 



COMPRESSED AIR PRODUCTION. 



to twenty-five atmospheres, will register 367.5 pounds above a vacuum 
(352.8 pounds gauge pressure), will occupy a volume of 0.1020 cubic foot, 
will have a temperature of 864 degrees, and the total increase of tempera- 
ture is 804 degrees. 

These tables apply to dry air only. The effect of moisture will vary 
the figures of temperature and to some extent will affect the pressures, 
but many useful deductions may be drawn from the tables. It is seen 
for instance by studying table 1 that a volume of dry air will be doubled if 



Tabte 2. — Heat 


PRODUCED BY COMPRESSION 


OF AIR. 




Pressure. 


a 


at 


'o 


■/. 






■2 


5 v 


V <u 


u 






O 


^ U-u W> 


rt 3 10 


V 




Pounds per 


S-J 




11 *J D 

i- Cu H 




Pounds per 


Sq. Inch 


"" V 




= £ fctf 


g 


Sq. Inch 


above the 


Vfu 


v ^ bCtfj 


CH O-H 


s 


above a 


Atm'sph're. 


§ 


t~ll 




< 


Vacuum. 


(Gauge 


"0 


v jz P 


** 






Pressure). 


> 


ft 




TOO 


14 70 


000 


1-0000 


60-0 


oo-o 


110 


16-17 


T47 


0-9346 


74-6 


14-6 


125 


18 37 


367 


0-8536 


94-8 


34-8 


1-50 


22-05 


7-35 


0-7501 


124-9 


64-9 


1-75 


25-81 


11-11 


6724 


15T6 


91-6 


200 


29-40 


14-70 


06U7 


175-8 


115*8 


2-50 


36-70 


2200 


05221 


218-3 


158 3 


3-00 


44-10 


29-40 


0-4588 


255-1 


1951 


350 


5P40 


36-70 


04113 


287-8 


227-8 


4-00 


58-80 


44-10 


0-3741 


317-4 


257-4 


5'00 


73 50 


58-80 


0-319 1 


360 4 


309-4 


6-00 


88-20 


73-50 


0-2806 


414-5 


354-5 


7-00 


102-90 


88-20 


0-2516 


4545 


394-5 


8-00 


117 60 


10290 


0-2288 


4906 


430-6 


900 


13230 


117-60 


0-2105 


523-7 


463-4 


1000 


147-00 


133-80 


0-1953 


554-0 


494-0 


15-00 


220*50 


205-80 


0-1465 


6810 


621-0 


2000 


294 00 


279-80 


0T195 


781-0 


72P0 


25-00 


367 50 


352-80 


0-1020 


864-0 


804-0 



its temperature is increased about 500 degrees, and conversely, of course, 
if the volume remains constant an increase of about 500 degrees in tem- 
perature will double the pressure. The addition of moisture serves to in- 
crease these figures, because moisture increases both the specific heat 
and the heat conducting capacity of the air. 

The thermal results of air compression and expansion are shown by 
the accompanying diagram (Fig. 2 — Frank Richards). Both the tem- 
perature of the air and its volume are shown at different stages of com- 
pression. The simplest application of this diagram is that which gives 
the gauge pressure represented at different points of the stroke. This is 
shown in the vertical lines. But in compressing air we produce heat, 
and it is important to know the temperature at any given pressure, also 
the relative volume. All of these are shown in the diagram. The initial 
volume of air equal to 1 is taken and divided into ten equal parts. Each 
division between two horizontal lines, shown by the figures at the right, 
representing one-tenth of the original volume. 

The vertical and horizontal lines are the measures of volumes, pres- 
sures and temperatures. The figures at the top indicate pressures in at- 



IO 



COMPRESSED AIR PRODUCTION. 



mospheres above a vacuum ; the corresponding figures at the bottom de- 
note pressures by the gauge. At the right are volumes from one-tenth to 
one. At tne left are degrees of temperatures from zero to 1,000 degrees 
Fahrenheit. The two curves which begin at the upper left hand corner 
and extend to the lower right are the lines of compression. 

The upper one being the "Adiabatic" curve, or that which represents 
the pressure at any point on the stroke with the heat developed by com- 
pression remaining in the air; the lower is the "Isothermal/' or the pres- 



N. *( ^ ^ ^ >» «s <fr 




FIG. 2. 

sure curve uninfluenced by heat. The three curves which begin 
at the lower left hand corner and rise to the right, are heat 
curves and represent the increase of temperature corresponding 
with different pressures and volumes, assuming in one case that the tem- 
perature of the air before admission to the compressor is zero, in another 
sixty degrees, and in another one hundred degrees. 

Beginning with the adiabatic curve, we find that for one volume of air, 
when compressed without cooling, the curve intersects the first vertical 
line at a point between 0.6 and 0.7 volume, the gauge pressure being 14.7 
pounds. If we assume that this air was admitted to the compressor at a 
temperature of zero, it will reach about 100 degrees when the gauge pres- 
sure is 14.7 pounds. We find this by following down the first line inter- 
sected by the adiabatic curve to the point where the zero heat curve inter- 
sects this same line, the reading being given in figures to the left imme- 
diately opposite. If the air had been admitted to the compressor at 60 de- 
grees, it would register about 176 degrees at 14.7 pounds gauge pressure. 
If the air were 100 degrees before compression, it would go up to about 
230 degrees at this pressure. Following this adiabatic curve until it inter- 
sects line No. 5, representing a pressure of five atmospheres above a 
vacuum (58.8 lb. gauge pressure), we see that the total increase of tem- 



COMPRESSED AIR PRODUCTION. I I 

perature on the zero heat curve is about 270 degrees; for the 60 degree 
curve it is about 370 degrees, and for the 100 degree curve it is about 435 
degrees. The diagram shows that when a volume of air is compressed 
adiabatically to 21 atmospheres (294 lb. gauge pressure) it will occupy a 
volume a little more than one-tenth; the total increase of tempera- 
ture with an initial temperature of zero is about 650 degrees; 
with 60 degrees initial temperature it is 800 degrees, and with 
100 degrees initial it is 900 degrees. It will be observed that 
the zero heat curve is flatter than the others, indicating that when free air 
is admitted to a compressor cold, the relative increase of temperature is less 
than when the air is hot. This points to the importance of low initial tem- 
perature. It is plain that a high initial temperature means a higher temper- 
ature throughout the stroke of a compressor. The diagram gives the loss 
of temperature during compression from initial temperatures of o deg., 60 
deg., 100 deg. If we compare the compression line from zero with the 
compression line from 100 deg., we observe that in compressing the air 
from, say 1 atmosphere to 10 atmospheres, the original difference which at 
the start was only 100 deg., has now been about doubled; that is, it has 
reached 200 deg., and in carrying the compression to 20 atmospheres the 
difference now becomes about 250 deg. Each horizontal division repre- 
sented by the figures at the left is equal to 100 deg., and the space be- 
tween any two adjacent horizontal lines may be subdivided into 100 equal 
parts representing 1 deg. each. 

Where there is a system of cooling the air during compression, the 
lines on the indicator cards can be traced between the adiabatic and iso- 
thermal curves on the diagram. In practice the best compressors show a 
line about midway between these two curves. Compressors using a 
spray of water for cooling show a pressure line a little nearer the isother- 
mal, but for reasons which will be referred to later on, spray compressors 
are now but little used. 

For all practical purposes in using this diagram it is best to follow 
the adiabatic curve in all determinations, except where the exact pres- 
sure line is known. This diagram will be found convenient to those who 
are called upon to figure the pressure at different points in the stroke 
of an air compressor, and it points out the common error of neglecting 
to take into consideration in one's figures the fact that, at the beginning 
of the stroke, one atmosphere in volume already exists. Beginning at 
the upper left har.d corner, the adiabatic pressure curve intersects the first 
vertical line at that point in the stroke when the pressure on the gauge 
will register 14.7 pounds. 

The next vertical line shows where the gauge reaches 29.4 pounds, 
and it is evident here that the piston of an air compressor travels much 
farther in reaching 14.7 pounds than in doubling that pressure or in 
reaching 29.4 pounds; thus an air compressor is an engine of unevenly 
distributed resistance. During the early stages of the stroke it has a 
slowly accumulating load to carry, while later on this load is multiplied 
very rapidly. This is one of the reasons for heavy fly wheels in air com- 
pressors. 



12 



COMPRESSED AIR PRODUCTION. 



Figure 3 is a graphic diagram drawn for the purpose of illustrating 
the fact that the power which is contained in any volume of air at a given 
pressure is dependent upon its distance in temperature above the absolute 
zero, and that there is as much power in a pound of air at fifteen pounds 
gauge pressure and 60 degrees temperature as there is in one pound of air 
at 100 pounds gauge pressure and 60 degrees temperature. One pound, 
or thirteen cubic feet, of air at fifteen pounds pressure and 60 degrees 
temperature, is represented by the space C. The available power in this 
air is 21,469 foot pounds. By available power is meant the amount of 
power which can be utilized when this air is expanded adiabatically to 
atmospheric pressure. The diagram shows that when such pressure is 
reached the temperature will be — 57 degrees Fahr. There still remains 
in this air a certain amount of intrinsic energy, and the diagram and fig- 






"E.J& £ sV.w fe-v o vy. 



ATm.ospKeirv.e' YAn& -. >-5; 



!46t°»b6olute Zeyo. 




iVre&au-re. ••*• '* »B. To loo ^^ Grange Press utc 



A. To \5^ )S Gravg,e ?re&au-re.. 

^^?_6o°«riij xl8i^ = JUA&9 Pt.Pa>| 



4.B4-°-60 o =4-14-°Xl83.s« 77,804., £. ft.fi.1. 



cr. 



froTtvW l,14, (ia»)« ?Tets,«Te. JO. ftom. loo'H<} AU rfe Pressiwe. 
-57*+6o°-ll7 , xifcV= 2I,46« FfcfAs, -a64 +6o°~«4.VlBSri 77,00* »lWi. 
Trotn 6o° «\: J\Trro»sbK« < rVc »»M»uTt. 



to JllifcOluTle ItTo. 



4-6 1°- ?7 o »4.04-°X 1 aa* m ^.l^.' FfcWk.'F. _4-»l-S64. , -97")ll»iTH J7.799 ft 
Tot*\.Ci»niE- 95.60* " Fk.WU- Total D...A f. 05,60a Ft 



«.Pi«. 



Fig. 3. 

ures show that this energy is equal to 74,134 foot pounds. This added 
to the available energy gives us 95,603 foot pounds as the whole energy 
contained in one pound of air at fifteen pounds pressure and 60 degrees 
temperature. 

D represents one pound of air at 100 pounds pressure and 60 degrees 
temperature. Its available energy is 77,804 foot pounds, and its intrinsic 
energy is i/,799 foot pounds, or the total energy is 95,603 foot pounds, 
which is exactly equal to the case just cited. 



COMPRESSED AIR PRODUCTION. 13 

Theae figures show the correctness of that thermodynamic law, which 
states that the power of any elastic gas is in direct proportion to its height 
of fall. So long as the temperature is above the absolute zero, there is 
as much powei in the same body of air when expanded adiabatically from 
a moderate temperature to an extremely low one, as when expanded from 
a high temperature to a moderate one, and this offers to some extent a 
limitation to that system of reheating which increases the volume without 
at the same time increasing the pressure. 

The development of heat when air is compressed is, perhaps, the 
best illustration of the acknowledged thermo-dynamic principle that work 
and heat are interchangeable unit for unit. When air is compressed all 
the work done in compression is converted into heat. This heat is capa- 
ble of being converted back again into power. But the question is fre- 
quently asked, if it be true that the power applied in the steam cylinder 
of an air compressor is all converted into heat in the air cylinder, how is it 
that power still remains in the compressed air after the heat has been 
lost through transmission? 

In order to get a clear understanding of this we must know that 
air is a power in itself before compression: that it contains a certain 
capacity for work due to its elasticity. It is not, however, in a condition 
available for work until compressed. This energy is made available by 
giving it a height of fall which is represented by a difference in pressure 
between the compressed air on one side and the free air on the other. 

If we box up free air at any given temperature and under normal 
atmospheric conditions, we have within the enclosure a well defined 
amount of energy. We cannot use it to perforin work unless the pressure 
outside is less than that inside the enclosure. This may be accomplished 
by placing the closed vessel in the rarified atmosphere, such as exists at 
altitudes, but in any case there is a well denned quantity of intrinsic 
energy within the air itself, the limit being measured by the height of 
fall between the free air in the vessel and absolute zero. 

Compression as now practiced only serves the purpose of placing 
the natural power which we have in the air into a condition which makes 
it possible for us to utilize it. This points to an undeveloped science in 
the use of compressed air. Inasmuch as we lose all the power expended 
in compression and yet have a capacity for useful work equal to from 
30 to 50 per cent, of that power, it is plain that there are possibilities 
in the science which are now misunderstood and not realized. 

It is theoretically possible to realize out of compressed air more 
power than was expended at the compressor. This has been shown in 
Fig. 1, but that this statement might not be confused with perpetual 
motion theories the sketch shown in Fig. 4 has been prepared. 

Sulphuric acid in its concentrated form will, when exposed in an 
open dish, absorb moisture from the atmosphere to the extent of about 
double its weight. A hypothetical assumption is made of a pump ar- 
ranged to discharge sulphuric acid in the direction shown by the arrow 
to an open dish, elevated (say) 100 feet. 



14 



COMPRESSED AIR PRODUCTION. 



The amount of power necessary to discharge a certain quantity of 
sulphuric acid into this dish is exactly equal to the power which the sul- 
phuric acid is capable of giving out when falling back again, less the fric- 
tion of the pump, leakage, etc. Now, let us assume that the sulphuric 
acid in the open dish remained there long enough to absorb moisture 
from the atmosphere until its weight has been doubled; it will thus ob- 
viously have twice the amount of power in falling back again, and if the 
friction and leakage losses were not too great it will be capable of driving 
the pump and of returning an equivalent volume of the concentrated acid 




Fig. 4. 

to the dish. If the same acid is used over again, the moisture must be 
driven out, and lamps are shown in the sketch provided for this purpose. 
The analogy between this hypothetical case of sulphuric acid and one of 
compressed air is that, as with acid we may draw power from moisture 
which is contained in the air, so with compressed air may we draw upon 
the intrinsic heat energy of the atmosphere. 

If it were practicable to compress air isothermally — that is, without 
heat — we might illustrate the point referred to in the foregoing by placing 
an air compressor in a cold room, or what would amount to the same 
thing, taking in cold air to the cylinder of the compressor, this air being 
taken, for instance, from a cold storage room, and on being compressed 
the temperature being maintained at the initial point. This cold com- 
pressed air might then be led in pipes placed on the surface of the ground 
and exposed, say, to the hot sun, so that when used its temperature might 
be largely increased above that at the compressor. 



COMPRESSED AIR PRODUCTINN. 15 

We would here have a case of reheating by natural conditions, and 
the possibility of obtaining more power at the motor than was expended 
at the compressor is made apparent. 

All this appears to be but theory, yet the value of the argument lies in 
the fact that it points to what may be accomplished in the future and to 
the importance at present of low initial temperature at the compressor 
and high temperature at the m-tor. It is a common thing to see air 
compressors at work in engine rooms drawing the air into the cvlinder at 
the temperature of the engine room, which, in many instances, especially 
in winter, is 50 degrees higher in temperature than that of the air on the 
outside. Even where air compressors are used with concentrated inlets 
made for the express purpose of being connected with the atmosphere 
from the outside, this question of economy by low initial temperature is 
frequently neglected. For every 5 degrees by which the initial tempera- 
ture of the intake air is lowered, there is a gain of one per cent, in volume. 

It is not a difficult matter to construct an air compressor plant where 
the intake air is made to pass over refrigeration pipes. In cities where 
ice-making plants are in operation, the best point to place the air com- 
pressor is alongside the ice-making machines, the combination of the two 
industries being advisable. 

On the other end of the line reheating is of great importance. A per- 
fect reheater has not yet been found, though at this time reheaters are in 
use which give practical results. It has already been demonstrated that 
compressed air may be increased in temperature and thus proportionately 
increased in volume and efficiency by the application of heat in a very 
economical manner, the quantity of coal consumed in proportion to the 
gain in economy being very small. 

Mr. Robert Hardie, in his experiments with a pneumatic motor using 
a hot water reheater, has figured the cost of reheating at one-eighth the 
coal required at the compressor. Professor Haupt, referring to this heat- 
er, makes the following statement: 

"The power required to compress 1,000 cubic feet of free air to 2,000 
pounds per minute would be 400 horse-power, consuming 1,200 pounds 
of coal per hour at a cost of $1.80 (at $3 per ton), and the cost of reheat- 
ing would not exceed 22 cents to double the work performed. That these 
statements are not simply theoretical deductions have been proved by- 
actual tests. The Rome air motor when using a reheater ran fourteen 
miles on a consumption of 308 cubic feet of free air per mile. When the 
air was not reheated, the consumption of air per mile was 661 cubic feet." 

The first loss in air compression, that due to the fact that the heat 
produced cannot be maintained, is an unavoidable loss. The second loss 
may be called the influence of the heat of compression, and is due to the 
fact that this heat increases ihe relative volume of the air and resists com- 



l6 COMPRESSED AIJj^ PRODUCTION. 

pression. This heat of compression has. long been the bete noire of air 
compressor builders. At first it seriously affected the valves and pack- 
ing, and this served as an argument in favor of injecting water into the 
cylinder, the claim of manufacturers being that by keeping down the heat 
of compression repairs would be less and accidents due to the destruction 
of parts by heat would be avoided. 

The injection of water into the air cylinder is usually known as the 
Colladon idea. Compressors built o^i this system have shown the high- 
est isothermal results. It is plain that the injection of cold water in the 
shape of a finely divided spray directly into the air during compression 
will lower the temperature to a greater degree than to simply surround 
the cylinder and parts by water jackets. 

Two systems are in use by which it is attempted to absorb the heat 
during compression. These systems divide air compressors into two 
classes — 

(i) Wet compressors. 

(2) Dry compressors. 

A wet compressor is one which introduces water directly into the 
cylinder during compression. 

A dry compressor is one which admits no water to the air during 
compression. 

Wet compressors may be subdivided into two classes — 

(1) Those which inject water in the form of a spray into the cylin- 
der during compression. 

(2) Those which use a water piston for forcing the air. into confine- 
ment. 

The advantages of water injection during compression are as fol- 
lows: 

(1) Low temperature of air during compression, hence a reduced 
mean resistance and a saving of power. 

(2) Increased volume of air per stroke, due to filling of clearance 
spaces with water, and to a cold air cylinder. 

(3) Low temperature of air-immediately after compression, thus con- 
densing moisture at the air receiver. 

(4) Low temperature of cylinder and valves, thus maintaining pack- 
ing, etc. 

(5) Economical results due to compression of moist air. (See Table 

No. 4.) 

The first advantage is by far the most important one, and is really the 
only excuse for water injection in air compression. 

The percentage of work of compression (dry air) which is converted 
into heat and lost when no cooling system is used is as follows : 
Compressing to 2 atmospheres, loss 9.2 per cent. 
Compressing to 3 atmospheres, loss 15.0 per cent. 



COMPRESSED AIR PRODUCTION. 



17 



Compressing to 4 atmospheres, loss 19.6 per cent. 

Compressing to 5 atmospheres, loss 21.3 per cent. 

Compressing to 6 atmospheres, loss 24.0 per cent. 

Compressing to 7 atmospheres, loss 26.0 per cent. 

Compressing to 8 atmospheres, loss 27.4 per cent. 
We see that in compressing air to five atmospheres, which is the 
usual practice, the heat loss is 21.3 per cent., so that if we keep down the 
temperature of the air during compression to the isothermal line, we save 
this loss. The best practice in America has brought this heat loss down 
ot 3.6 per cent, (old Ingersoll Injection Air Compressors), while in 
Europe the heat loss has been reduced to 1.6 per cent. Steam-driven 
air compressors are usually run at a piston speed of about 350 feet per 
minute, or from 60 to 80 revolutions per minute of compressors of aver- 
age sizes, say 18 inches diameter of cylinder. Sixty revolutions per min- 
ute is equal to 120 strokes, or two strokes per second. An air cylinder 18 
inches in diameter filled with free air once every half second, and at each 
stroke compressing the air to 60 pounds, and thereby producing 309 de- 
grees of heat, is thus by means of water injection cooled to an extent 
hardly possible with mere surface contact. The specific heat of water 
being about four times that of air, it readily takes up the heat of com- 
pression. 

A properly designed spray system must not be confused with the 
numerous devices applied to air cylinders by means of which water is 
introduced. In some cases the water is merely drawn in through the 
inlet valves. In others it passes through the centre of the piston and 
rod, coming in contact with the interior walls of the air cylinder between 
the packing rings. Introducing water into the air cylinder in any other 
way, except in the form of a spray, has but little effect in cooling the air 
during compression. On the contrary, it is a most fallacious system, be- 
cause it introduces all the disadvantages of water injection without its 
isothermal influence. Water, by mere surface contact with air, takes up 
but little heat, while the air having a chance to increase its temperature, 
absorbs water through the affinity of air for moisture, and thus carries 
over a volume of saturated hot air into the receiver and pipes, which on 
cooling (as it always does in transit to the mine), deposits its moisture 
and gives trouble through water and freezing. It is therefore of much 
importance to bear in mind that unless water can be introduced during 
compression to such an extent as to keep down the temperature of the air 
in the cylinder, it had better not be introduced at all. 

If too little water is introduced into an air cylinder during compres- 
sion, the result is warm, moist air, and if too much water is used it results 
in a surplus of power required to move a body of water which renders no 
useful service. 

The following table deduced from Zahner's formula gives the quan- 



i8 



COMPRESSED AIR PRODUCTION. 



tity of water which should be injected per cubic foot of air compressed in 
order to keep the temperature, down to 104 degrees Fahrenheit. 





Weight of Water 


Weight of Water 


Compression 

by atmosphere 

above 


to be injected at 
68° Fah. to keep 
the temperature 


to be injected at 
68° Fah. to keep 
the temperature 


at io4°Fah.in lbs. 


at 104 Fah. in 


a vacuum. 


of water and per 


lbs of water for 




lbs. of free air. 


1 cu. ft. of free air 


2 


0.734 


0-056 


3 


1664 


0-089 


4 


1-469 


0113 


5 


1-701 


0131 


6 


1-891 


0145 


7 


3(63 


0158 


8 


3 304 


0-167 


9 


3 329 


0*179 


10 


3440 


0188 


11 


3 513 


0195 


13 


3 634 


0303 


13 


3719 


0-309 


14 


3798 


0315 


15 


3-871 


0233 



Experiments were made under the personal supervision of Prof. Den- 
ton at the Stevens Institute of Technology, to determine the relative 
effects in air compressors of water injection and water jackets. An ex- 
tended controversy had been carried on in the "Engineering- and Mining 
Journal" between the writer and others upon the question whether or not 
the injection of water reduced the temperature and increased the effi- 
ciency. It was claimed by some that the efficient indicator cards taken 
from certain injection compressors were not reliable because of leakage. 
It was only on such grounds that the proximity of the pressure line to the 
isothermal could be explained away. In the .Stevens Institute tests the 
greatest care was exercised to secure air-tight pistons and valves, and as 
these experiments were unbiased and in the hands of experts, the results 
may be accepted as conclusive so far as indicating isothermal economy by 
an efficient system of injection. Fig. 6 herewith represents graphically 
the results obtained at the Stevens Institute.* No clearance is shown, 
because the purpose of the illustration is to show the comparative effect 
of the various methods of cooling. The air was compressed to 150 
pounds gauge pressure, the work done in each case being represented by 
the area between the pressure curve and the rectangular lines. 

The pressure curve is determined in each case by the formula — 



(v) B 



In the case of isothermal compression (assuming no heat produced), 
the exponent N=i. In adiabatic compressidn (the full effect of the heat 
of compression being available), N= 1.408. It is obvious that the value 



From a paper by Mr. 11. A. Parke. 



COMPRESSED AIR PRODUCTION. 



19 



of the exponent N will vary between the two points 1, and 1.408. From 
a large number of indicator cards taken at the Stevens Institute the fol- 
lowing values were shown: 

Water jacket N=i.35 

Water jet injection N=i.33 

Water spray injection N— 1.25 

It has been demonstrated by experiments made in France that the 
power required to compress dry air has been prepared from the data of M. 




Fig. 6. 

Mallard, and shows that for five atmospheres the work expended in com- 
pressing one pound of dry air is 58,500 foot pounds, while that for moist 
air is 52,500 foot pounds. In expansion also moisture in the air adds to 
the economy, but in both cases the saving of power is not great enough 
to compensate for the many disadvantages due to the presence of water. 
Mr. Norman Selfe, of the Engineering Association of N. S. W., has 
compiled a table which shows some important theoretical conditions in- 
volved in producing compressed air. 



20 



COMPRESSED AIR PRODUCTION. 



P 
►4 
O 

> H 

P 

fc ° 

3 5 



£ 5 s 

a ;> en 



-2 .ji 

w w <J 

a p-0 

o<-> « 
o c 
fc o 



a3 



^S 



aiy o; J3;bav jo 33b;u30J3 j 



eo -in -* i!o 6 50 



•uots 

-ssjdxno3 nt pasn si j3}bm. 

jt amiBj^dmaj, l" BU J J 



SfSSo 



riSti o'S i! 2 rt « 
^ « — Si o*o «W 



•° ■£ k/ 






TO U 

Si 3 

2 * 



° S " S « 

2«^£$ 



u 

V Q 



M .2 



ll-sll 

a fe ^o 






a 

o V 

uSi. • 

wH.2 
<n u 

Si - * 
£ to 



£ 

OS 

■a" 



-mTJ 



- « .2 o S 



o !»: 



a S.SoS 



•saaaiidsoia^v "I uoisusj. 



M3M3 ... I 

<^OfOt-C»~3 ■ • • I 

nroirtcot-o: ; ; ; ! 



6660666 



I i 






O N CO -* »0 SO t- CO 00 Ci 10 
r-l r-i r-l rH r-( i— I rH rH r-4 Ah *~" ' i 



ootcNO^aoiio 

©WtDMX*ll»OMH 



>p ■* 5D CO U3 C^l 03 

Q>*o6>'oib©?0C0*'l3S 

SSaoeosooicxi^tOaocs 

HHHN«N«5l 






l*-i 



«r- ec 1- >— t -*• t- 3j 

HHNNNN 



|»0>-*OHN900 
i—( lO i- <M CO iO 'rtffi 

666606666 









^HMSrOe^'MrHrH — rHrH 

6666666666 



t-H «^ OS ■* W5 CO t- oo o> o ^ 



COMPRESSED AIR PRODUCTION. 2 1 

There are many serious objections, however, to the use of water with- 
in the air cylinder. These objections are so serious that it has been found 
to be the best practice to suffer the heat loss during compression, and thus 
simplify the apparatus. Some of the objections may be stated as follows: 

1. The mechanical difficulties involved in introducing the water into 
the cylinder so intimately mixed with the air during compression as to 
reduce the temperature of compression immediately when produced. 

2. Impurities in the water, which, through both mechanical and 
chemical action, destroy exposed metallic surfaces. 

3. Wear of cylinder, piston and other parts, due directly to the fact 
that water is a bad lubricant; and as the density of water is greater than 
that of oil, the latter floats on the water and has no chance to lubricate 
the moving parts. 

4. Wet air arising from insufficient quantity of water and from in- 
efficient means of ejection. 

5. Mechanical complications connected with the water pump, and 
the difficulties in the way of proportioning the volume of water and its 
temperature to the volume, temperature and pressure of the air. 

6. Loss of power required to overcome the inertia of the water. 

7. Limitations to the speed of the compressor, because of the lia- 
bility to break the cylinder head joint by water confined in the clearance 
spaces. 

8. Absorption of air by water. 

Before the introduction of condensing air receivers, wet air resulting 
in freezing was considered the most serious obstacle to water injection; 
but this difficulty no longer exists, as experience has demonstrated that 
a large part of the moisture in compressed air may be abstracted in the 
air receiver. Even in the so-called dry compressors a great deal of moist- 
ure is carried over with the compressed air, because the atmosphere is 
never free from moisture. This subject will be referred to more fully 
when treating of the transmission of compressed air. 

A serious obstacle to water injection, and that which condemns the 
wet compressor, is the influence of the injected water upon the air cylin- 
der and parts. Even when pure water is used, the cylinders wear to such 
an extent as to produce leakage and to require reboring. The limitation 
to the speed of a compressor is also an important objection. The claim 
made by some that the injected water does not fill the clearance spaces, 
but is aerated, does not hold good, except with an inefficient injection 
system. 

Whether it be water or spray which occupies the clearance space, it 
is impossible for air and spray to occupy the same place at the same time. 

The writer has increased the speed of an air compressor (cylinders 12 
in. and 12 in. by 18 in., injection ten revolutions per minute) by placing 
his fingers over the orifice of the suction pipe of the water pump. The 
boiler pressure remained the same, the cut-off was not changed and the 
air pressure was uniform, hence this increase of speed arose from the fact 
that the water was restricted and the clearance spaces were filled with 
compressed air, which served as a cushion or spring. While the volume 



2 2 (COMPRESSED AIR PRODUCTION. 

of compressed air furnished by this compressor would be somewhat re- 
duced by the restriction of the water, yet the increase in speed which was 
obtained without any increase of power, fully compensated lor the clear- 
ance loss. 

Unless the water of injection can be used efficiently as a cooling 
agent, its value for clearance does not compensate for the disadvantages 
attending its use. In some of the early types of air compressors water 
was introduced through the inlet valves during the suction stroke; but this 
is an objectionable plan, because it has little effect on the heat produced 
until the discharge of the air, and furthermore, there is the danger of in- 
troducing too much water, and thus reducing the volume of air and en- 
dangering the cylinder heads. 

The presence of moisture in the air reduces the heat loss, hence, as 
shown by Table No. 4, less power is required to compress moist than dry 
air. It is not necessary to inject water during compression in order to 
gain this advantage, as the atmosphere is usually moist. The presence of 
moisture in the air has an important bearing upon the compression, trans- 
mission and use of air. Before compressed air became generally used, its 
value was thought to be prohibitive, mainly because it was said that the 
air would freeze. This freezing was, of course, nothing more than the 
formation of ice due to the presence of moisture in the air, this moisture 
having been first deposited in the shape of water by expansion and cool- 
ing, and afterwards, the temperature going down below the freezing point, 
it became ice. This has some time since ceased to be a serious matter, 
and on the whole the presence of the moisture has been found to be more 
beneficial than otherwise, because by increasing the specific heat of the 
body with which it is in contact, it reduces the temperature during com- 
pression and tends to increase it during expansion. In transmission it is 
simply necessary to keep the temperature from falling below the dew 
point, or to put in suitable receivers for draining the pipes. Where re- 
heaters are used, the presence of moisture is decidedly advantageous. 

The amount of moisture in the atmosphere varies with the climate. 
Air is never perfectly dry ; never, except in rare instances, does it contain 
less than 25 per cent, of the moisture necessary to saturate it. It is not an 
uncommon thing to read in the meterological reports in the newspapers 
during the summer that the moisture during an oppressively hot day 
reached 98 per cent., and even 99 per cent. In winter it is usually 80 or 
85 per cent. At 65 per cent, we consider it moderately dry; 50 per cent, 
being commonly called dry air. 

Otto Van Guericke invented the air-pump in 1650. In 1753 Holl 
used an air engine for raising water. At Ramsgate Harbor, Kent, in the 
year 1788, Smeaton invented a "pump" for use in a diving apparatus. 
In 1 85 1, William Cubitt, at Rochester Bridge, and a little later an engi- 
neer, Brunei, at Saltash, used compressed air for bridge work. In 1852, 
Colladon patented the application of compressed air for driving machine 
drills in tunnels. The first notable use of compressed air is due to Prof. 
Colladon, of Geneva, whose plans were adopted at the Mont Cenis tunnel. 



COMPRESSED AIR PRODUCTION. 



23 



M. Sommeiller developed the Colladon idea and constructed the com- 
pressed air plant illustrated in Fig. 7. 

The Sommeiller compressor was operated as a ram, utilizing a 
natural head of water to force air at 80 pounds pressure into a receiver. 
The column of water contained in tne long pipe on the side of the hill 
was started and stopped automatically, by valves controlled by engines. 
The weight and momentum of the water forced a volume of air with 
such shock against a discharge valve that it was opened and the air was 
discharged into the taipk; the valve was then closed, the water checked; a 
portion of it was allowed to discharge and the space was filled with air, 
which was in turn forced into the tank. The efficiency of this compressor 
was about 50 per cent. 




ffl^^^J 



fig. 7.-sommeitter air compressor used at the mt. cents tunnel. 

At the St. Gothard tunnel, begun in 1872, Prof. Colladon first intro- 
duced the injection of water in the form of spray into the compressor 
cylinder to absorb the heat of compression. 

Fig. 8 illustrates the air cylinder of the Dubois-Francois type of com- 
pressor, which was the best in use about the year 1876. This com- 
pressor w r as exhibited at the Centennial Exposition and was adopted by 
Mr. Sutro in the construction of the Sutro tunnel. A characteristic feat- 
ure seems to be to get as much water into the cylinder as possible. The 
water which flooded the bottom of the cylinder arose from the voluminous 
injection; this water was pushed into the end of the cylinder and some of 
it escaped with the air through the discharge valve. 

An improved pattern of this compressor is shown in Fig 9. 

The first air compressor used on a large scale for practical work in 
America is shown in Fig. 10. 



H 



COMPRESSED AIR PRODUCTION. 



This machine was used at the Hoosac Tunnel, being built a little 
prior to 1866. The design was made under the direction of the Massa- 
chusetts State Commission, of which Mr. John W. Brooks was chairman 




FIG. 8.— DUBOIS & FRANCOIS, 1876. 



and Mr. Thomas Doane, chief engineer for tunnel construction. It is be- 
lieved that Mr. Doane deserves the largest share of credit for the inven- 
tion and development of this compressor, and it is to the credit of these 




FIG. 9.— DUBOIS & FRANCOIS, 1884. 



early designers to note that after the completion of the Hoosac Tunnel 
the compressor was transferred to the Marble Quarries, at Sutherland 
Falls, Vt. (now called Proctor), and that it has been used continuously up 



CCMPRESSED AIR PRODUCTION. 



25 



to the present time, compressing air to about 40 pounds to the square 
inch. Rock drills and channeling machines of modern construction are 
now using this air for quarrying the beautiful marble of Vermont. 

The first channeling machine was tried in this quarry perhaps with 
compressed air furnished by the Hoosac Tunnel compressor. 

The compressor is so simple that it may be readily understood by 
looking at the plan. It consists of 4 horizontal air cylinders, the pistons 
of which are propelled by a turbine wheel. The cylinders are single 
acting, the air being admitted through poppet valves placed in the piston. 
Each cylinder is 13 in. in diameter by 20 in. in stroke. It was originally 
intended for a speed of 120 revolutions per minute, but it has been run 




FIG. 



over 70 revolutions. The cooling arrangement applied to this com- 
pressor was simply an injection of water through the inlet valves into the 
cylinders, though since its use at Hoosac Tunnel, injection has been aban- 
doned, and a simple stream of water from a jet is allowed to play upon the 
cylinders. 

These illustrations are interesting from an historical point of view, as 
indicating the line of thought which early designers of air compressing 
machinery followed. As the necessity for compressed air power grew, 
inventors turned their attention to the construction of air-compressing en- 



COMPRESSED AIR PRODUCTION, 



ghies that would combine efficiency with light weight and economy of 
space and cost, the trade demanded compressors at inaccessible locali- 
ties, and in many cases it was preferred to sacrifice isothermal results to 
simplicity of construction and low cost. 




FIG. II.— DIRECT COMPRESSION ILLUSTRATED IN THE STRAIGHT 
LINE AIR COMPRESSOR IN WHICH THE MOMENTUM OF THE 
FLY WHEEL EQUALIZES THE PRESSURE- 

It is evident that an air compressor which has the steam cylinder and 
the air cylinder on a single straight rod will apply the power in the most 
direct manner, and will involve the simplest mechanics in the construc- 
tion of its parts. It is evident, however, that this straight line, or direct 
construction, results in an engine which has the greatest power at a time 








^} elc\v ev^ne. 

n c\,l. ^ 

S>Te&w. 



* 








1 il 


1 — CI 




*7 


1 







ft\<l\ 

S"\ec\Yri 



s >^-' 




when there is no work to perform. At the beginning of the stroke, 
steam at the boiler pressure is admitted behind the piston, and as the air 
piston at that time is also at the initial point in the stroke, it has only 



COMPRESSED AIR PRODUCTION. 27 

free air against it. The two pistons move simultaneously as the resist- 
ance in the air cylinder rapidly increases as the air is compressed. To get 
economical results it is, of course, necessary to cut off in the steam 
cylinder so that at the end of the stroke, when the steam pressure is low, 
as indicated by the dotted line (Fig. n), the air pressure is high, as 
similarly indicated in the other cylinder. The early direct-acting com- 
pressor used steam at full pressure throughout the stroke. The Westing- 
house pump applied to locomotives, is built on this principle, and those 
who have observed it work have perhaps noticed that its speed of stroke 
is not uniform, but that it moves rapidly at the beginning, gradually re- 
ducing its speed, and then seems to labor until the direction of stroke is 
reversed. This construction is admitted to be wasteful, but in some cases, 
notably that of the Westinghouse pump, economy in steam consumption 
is sacrificed to lightness and economy of space. 

Many efforts were made to equalize the power and resistance by con- 
structing the air compressor on the crank shaft principle, putting the 
cranks at various angles, and by angular positions of steam and air cylin- 
ders. Several types are shown in Fig. 12. 

Angular positions of the cylinder involve expensive construction and 
unsteadiness. Experience has proved that there is nothing in the appar- 
ent difficulty in equalizing the strains in a direct-acting engine. It is sim- 
ply necessary to add enough weight to the moving parts, that is, to the 
piston, piston rod, fly wheel, etc., to cut off early in the stroke and secure 
rotative speed with the most economical results and with the cheapest 
construction. It is obvious that the theoretically perfect air compressor 
is a direct-acting one with a conical air cylinder, the base of the cone 
being nearest the steam cylinder. This, from a practical point of view, 
is impossible. Mr. E. Hill, in referring to the fallacious tendencies of 
pneumatic engineers to equalize power and resistance in air compressors, 
says: 

"The ingenuity of mechanics has been taxed and a great variety 
of devices have been employed. It is usual to build on the pattern of 
presses which do their work in a few inches of the end of the stroke and 
employ heavy fly wheels, extra strong connections, and prodigious bed 
plates. Counterpoise weights are also attached to such machines; the 
steam is allowed to follow full stroke, steam cylinders are placed at awk- 
ward angles to the air-compressing cylinders and the motion conveyed 
through yokes, toggles, levers; and many joints and other devices are 
used, many of which are entire failures, while some are used with ques- 
tionable engineering skill and very poor results." 

Fig. 13 illustrates the theory of Duplex Air Compressors. The 
hydraulic piston or plunger compressor is largely used in Germany and 
elsewhere on the Continent of Europe, but the duplex may be said to be 
the standard type of European compressor at the present time. It is also 
largely used in America. Fig. 13 shows the four cylinders of a duplex 
compressor in two positions of the stroke. It will be observed that each 
steam cylinder has an air cylinder connected directly to the tail rod of its 



28 



COMPRESSED AIR PRODUCTION. 



piston, so that it is a direct-acting machine, except in that the strains are 
transmitted through a single fly wheel, which is attached to a crank shaft 
connecting the engines. In other words, a duplex air compressor would 
be identical with a duplex steam engine, except in that the air cylinders 
are connected to the steam piston rods. The result is, as shown in Fig. 
13, that at that point of the stroke indicated in the top section, the upper 
right hand steam cylinder, having steam at full pressure behind its piston, 
is doing work through the angle of the crank shaft upon the air in the 
lower left hand cylinder. At this point of the stroke the opposite steam 
cylinder has a reduced steam pressure and is doing little or no work, be- 
cause the opposite air cylinder is beginning its stroke. Referring now 
to the lower section, it will be seen that the conditions are reversed. One 
crank has turned the center, and that piston which in the upper section 
was doing the greatest work is now doing little or nothing, while the 
labor of the engine has been transferred to those cylinders which a mo- 
ment before had been doing no work. 



1 --J -.--1 1 . 


?--... I..!--- r- 







r 



■z& 



FIG. 13. 

This is the theory of the Duplex Compressor, but it can hardly be said 
to be true when applied in practice, because of the heavy fly wheel which 
is placed on the main shaft. This wheel takes up and equalizes the power 
imparted by the steam pistons, and in fact it is the fly wheel which really 
does the work of compression at or near the end of each stroke. Heavy 
fly wheels are therefore essential in order to produce the best results with 
duplex compressors. 

In the duplex pattern, the crank shafts being set quartering, as is the 
usual construction, the engine may be run at low speed without getting 
on the center. Each half being complete in itself, it is possible to detach 
the one when only half the capacity is required. 

Commercially the Duplex Compressor appeals to the trade in that 
one side, or a half duplex, is furnished with a fly wheel and outboard bear- 
ings designed for a complete duplex machine, so that at first, at a little 
more than half the expense, one side is erected and when more air is 
needed the capacity of the plant is doubled by adding the other half. 
Where large capacities are required the duplex will admit of larger air 
production with economical engines, and at less expense when the cost of 
the plant is considered in proportion to the volume of air furnished. 



COMPRESSED AIR PRODUCTION. 29 

Mr. John Darlington, of England, gives the following particulars of 
a modern air compressor of European type: 

"Engine, two vertical cylinders, steam jacketed, with Myer's expan- 
sion gear. Cylinders, 16*9 inches diameter, stroke 39-4 inches; com- 
pressor, two cylinders, diameter of piston 23.0 inches; stroke 39.4 inches; 
revolutions per minute, 30 to 40; piston speed, 39 to 52 inches per second; 
capacity of cylinder per revolution, 20 cubic feet; diameter of valves, viz., 
four inlet and four outlet, 5^ inches ; weight of each inlet valve, 8 lbs. ; 
outlet, 10 lbs.; pressure of air, 4 to 5 atmospheres. The diagrams taken 
of the engine and compressor show that the work expended in compres- 
sing one cubic meter of air to 4:21 effective atmospheres was 38,128 lbs. 
According to Boyle and Mariotte's law it would be 37,534 lbs., the differ- 
ence being 594 lbs., or a loss of 1 '6 per cent. Or if compressed without 
abstraction of heat, the work -expended would in that case have been 
48,158. The volume of air compressed per revolution was 0*5654 cubic 
meters. For obtaining this measure of compressed air, the work ex- 
pended was 21,557 pounds. 

"The work done in the steam cylinders, from indicator diagrams, is 
shown to have been 25,205 pounds, the useful effect being 85^ per cent, 
of the power expended. The temperature of air on entering the cylinder 
was 50 deg. Fah.; on leaving, 62 deg. Fah., or an increase of 12 deg. Fah. 
Without the water jacket and water injection for cooling the temperature 
it would have been 302 deg. Fah. The water injected into the cylinders 
per revolution was 0.81 gallons." 

We have in the foregoing a remarkable isothermal result. The heat 
of compression is so thoroughly absorbed that the thermal loss is only 1 '6 
per cent.; but the loss by friction of the engine is 14-5 per cent., and the 
net economy of the whole system is no greater than that of the best 
American dry compressor, w r hich loses about one-half the theoretical loss 
due to heat of compression, but which makes up the difference by a low 
friction loss. The wet compressor of the second class is the water piston 
compressor, Fig. 14. 

The illustration shows the general type of this compressor, though it 
has been subject to much modification in different places. In America 
a plunger is used instead of a piston, and as it always moves in water, the 
result is more satisfactory. The piston, or plunger, moves horizontally 
in the lower part of a U-shaped cylinder. Water at all times surrounds 
the piston and fills alternately the upper chambers. The free air is ad- 
mitted through a valve on the side of each column and is discharged 
through the top. The movement of the piston causes the water to rise 
on one side and fall on the other. As the water falls the space is occu- 
pied by free air, which is compressed when the motion of the piston is 
reversed, and the water column raised. The discharge valve is so propor- 
tioned that some of the water is carried out after the air has been dis- 
charged. Hence there are no clearance losses. 

The chief claim for this w r ater piston compressor is that its piston is 
also its cooling device, and that the heat of compression is absorbed by 



3° 



COMPRESSED AIR PRODUCTION. 



the water. So much confidence seems to be placed in the isothermal feat- 
ures of this machine that usually no water jacket or spray pump is ap- 
plied. Mr. Darlington, who is one of the stanch defenders of this class 
of compressors, has found it necessary to introduce "spray jets of water 
immediately under the outlet valves," the object of which is to absorb a 
larger amount of heat than would otherwise be effected by the simple con- 
tact of the air with the water-compressing column. Without such spray 
connections, it is safe to say that this compressor has scarcely any cooling 
advantages at all, so far as air cooling is concerned. Water is not a good 
conductor of heat. In this case only one side of a large body of air is ex- 
posed to a water surface, and as water is a bad conductor, the result is that 
a thin film of water gets hot in the early stage of the stroke, and little or 
no cooling* takes place thereafter. The compressed air is doubtless cooled 




FIG. 14— HYDRAULIC AIR COMPRESSOR. 

before it gets even as far as the receiver, because so much water is tum- 
bled over into the pipes with it; but to produce economical results, the 
cooling should take place during compression. 

Water and cast iron have about the same relative capacity for heat 
at equal volumes. In this water piston compressor we have only one 
cooling surface, which soon gets hot, while with a dry compressor, with 
water jacketed cylinders and heads, there are several cold metallic sur- 
faces exposed on one side to the heat of compression, and on the other 
to a moving body of cold water. 

But the water piston advocate brings forward the question of speed. 
It is said that, admitting that the cooling surfaces are equal, we have in 
one case. more time to absorb the heat than in the other. This is true, 
and here we come to an important class division in air compressing ma- 
chinery high speed and short stroke as against slow speed and long 
stroke. Hydraulic piston compressors are subject to the laws that 
govern piston pumps, and are, therefore, limited to a piston speed of 



COMPRESSED AIR PRODUCTION. 3 1 

about ioo feet per minute. It is quite out of the question to run them at 
much higher speed than this without shock to the engine and fluctuations 
of air pressure due to agitation of the water piston. The quantity of heat 
produced — that is, the degree of temperature reached — depends entirely 
upon the conditions in the air itself, as to density, temperature and moist- 
ure, and is entirely independent of speed. We have seen that it is possible 
to lose 21 "3 per cent, of work when compressing air to five atmospheres 
without any cooling arrangements. With the best compressors of the dry 
system one-half of this loss is saved by water jacket absorption, so that we 
are left with about n per cent., which Lie slow moving compressor seeks 
to erase. We are quite safe in saying that the element of time alone in 
the stroke of an air compressor could not possibly effect a saving of more 
than half of this, or 5^ per cent. Now, in order to get this 5J per cent, 
saving, we reduce the speed of an air-compressing engine from 350 feet 
per minute to 100 feet per minute. We must, therefore, in one case have 
a piston area three and one-half times that of the other in order to get the 
same capacity of air, and in doing this we build an engine of enormous 
proportions, with heavy moving parts. W r e load it down with a large 
mass of water, which it must move back and forth during its work, and 
thus we produce a percentage of friction loss alone equal to twice or even 
three times the 5-^ per cent, heat loss which is responsible for all this ex- 
pense in first cost and in maintenance, but which really is not saved, after 
all, unless water injection in the form of spray also forms a part of the 
system. 

It is obvious that cost of construction and maintenance have much to 
do with the commercial value of an air compressor. The hydraulic piston 
machine not only costs a great deal more in proportion to the power it 
produces, but it costs more to maintain it, and it costs more to run it. It 
is not an uncommon thing to hear engineers speak of the hydraulic piston 
compressor as the "most economical" machine for the purpose, but that 
it is so "expensive" and takes up so much room, and requires such ex- 
pensive foundations that, unless persons are "willing to spend so much 
money," they had better take the next best thing, a high speed machine. 

The hydraulic piston compressor has one solitary advantage, and 
that is, it has no dead spaces. It was conceived at a time when dead 
spaces were very serious conditions. Valves and other mechanism con- 
nected with the cylinder of an air compressor w r ere once of such crude 
construction th^t it was impossible to reduce the clearance spaces to a 
reasonable point, and furthermore, the valves were heavy and so com- 
plicated that anything like a high speed would either break them or wear 
them out rapidly, or derange them so that leakages would occur. But 
we have now reduced inlet and discharge valves and all other moving 
parts connected with an air cylinder to a point of extreme simplicity. 
Clearance space is in some cases destroyed altogether by what is, as it 
were, an elastic air head which is brought into direct contact w T ith the 
piston. All this reduces clearance to so small a point that it has no in- 
fluence of any consequence, The moving parts are made extremely 
simple. 



32 COMPRESSED AIR PRODUCTION. 

Mr. Sturgeon, of England, has applied a most ingenious and success- 
ful inlet valve, which is opened and closed by the friction of the air piston 
rod through the gland. Mr. Sergeant in America has introduced the pis- 
ton inlet valve, which is opened and closed by its own inertia. We have, 
therefore, reached a point at which high speed is made possible. 

In the single or straight line compressor it is difficult to equalize 
power and resistance with long strokes. The speed will be jerky, and 
when slow the fly wheel rather retards than assists in the work of com- 
pression. This action tends to derange the parts and makes large bear- 
ings a necessity. The piston in a long stroke compressor travels through 
considerable space before the pressure reaches a point where the dis- 
charge valve opens, and after reaching that point it has to go on still fur- 
ther against a prolonged uniform resistance. This makes rotative speed 
difficult in single direct acting machines. During the early part of the 
stroke, the energy of the steam piston must be stored up in the moving 
parts, to be given out when the steam pressure has been reduced through 
an early cut-off. With a short stroke and a large diameter of steam cylin- 
der we are able to get steam economy or early cut-off and expansion 
without compounding. 

In compressors of the single or direct acting type with steam and 
air cylinders of equal diameter it is possible to obtain a pressure of air 
twice as great as the boiler pressure. This apparent enigma is made 
plain when it is understood that at the beginning of the stroke there is no 
resistance in the air cylinder. The steam end at this point has its great- 
est power, and the supply may be cut-off and the steam expanded in pro- 
portion to the pressure required in the air end, and the speed of the ma- 
chine. The indicator card shows a large volume and low pressure in the 
steam end, and a smaller volume and higher pressure in the air end, so 
that what is made up in the air card by high pressure is represented in the 
steam card by greater volume, and the area of one is nearly equal to that 
of the other. This can be seen by referring to Fig. n. 

If we omit the cut-off on the steam end the pressure, instead of fol- 
lowing the dotted lines, will be maintained at its maximum throughout 
the stroke, while the air pressure, or resistance, does not reach the steam 
pressure until the piston has passed the centre of the cylinder; hence if 
there is sufficient inertia in the moving parts, there will be no difficulty 
in getting an air pressure higher than that of the steam. 

CLEARANCE. 

The early designers of air compressors, as shown in the Dubois & 
Francois illustrations (Figs. 8 and 9), mention clearance loss in air com- 
pressors as a very serious matter. Even at the present time some air 
compressor manufacturers admit water through the inlet valves into the 
air cylinder, not so much for the purpose of cooling as to fill up the 
clearance spaces. A long stroke involving expensive construction is 
sometimes justified by the claim that a saving is effected by reduced clear- 
ance losses. 



COMPRESSED AIR PRODUCTION. $$ 

Clearance in a properly designed compressor is a loss of volume 
only, not a loss of power. Let us assume, for the sake of illustration, 
that we are compressing air with a machine which is provided with so 
efficient a cooling device that all of the heat of compression will be ab- 
sorbed as soon as produced. In other words, that we can compress air 
isothermally. In such a machine as this there will be a slight loss of 
power due to clearance space, because we would have a certain volume of 
air in the cylinder at each stroke, and upon which work had been done 
and heat produced, that heat having been absorbed and the air being re- 
tained in the cylinder. In other words, we would have a production and 
abstraction of heat, Which would represent power lost. Isothermal com- 
pression is practically impossible; hence we do not abstract the heat from 
the compressed air in the clearance space, but a large portion of this heat 
remains, and acts expansively upon the air, imparting its power to the 
piston at the moment of reversal of stroke. A reasonable clearance space 
behind the air piston serves a useful purpose in overcoming the inertia of 
the piston and moving parts acting like a spring at the end of each 
stroke. 

The clearance space in modern air compressors of the best design 
(including the counter-bore and discharge valve clearance) varies from 
.002 to .0094 of the volume of free air furnished by the cylinder. The vari- 
ation is somew'hat dependent upon the length of stroke of the machine. 
At 75 lbs. pressure, and making due allowance for increased volume 
of air due to heat, the clearance loss of volume varies from .01 to .047, 
or from one to five per cent, of the air when compressed. The actual 
space between the piston and the head at the end of the stroke being 
1-16 in. 

It must not be inferred that the designers of an air compressor may 
neglect the question of clearance; on the contrary, it is a very important 
consideration. If we have a large clearance space in the end of an air 
compressor which is used to compress air to high pressures, we may read- 
ily understand a condition of things that would result in no discharge of 
compressed air at all, because of too large a clearance space. The entire 
volume of the cylinder might be compressed and retained in the clear- 
ance space, and the compressor will take in no free air on the return 
stroke, because the clearance space air when expanded is sufficient to fill 
the cylinder at normal or atmospheric pressure. 

Lass in capacity of air compressors by clearance is in direct propor- 
tion to the pressure. 

Owing to the loss of capacity by clearance in high pressure com- 
pressors, it is important that the cylinders be compounded. By com- 
pounding the air in the cylinders the clearance loss is reduced because of 
the reduced density in the air in the clearance space. 

Builders of air compressors employ three methods to reduce the 
clearance loss: (1) By long strokes of piston, so that the percentage of 
cvlinder volume to that of clearance space is reduced to a minimum. (2) 

2 



34 COMPRESSED AIR PRODUCTION. 

By filling the space with water. (3) By not allowing the full reservoir 
pressure to accumulate in the clearance space above the inlet valve. 

The long stroke plan is the best for reducing clearance, except in ma- 
chines of the single type, where economical compression and rotative 
speed cannot be accomplished with a long stroke*. The use of water to 
fill clearance space has been referred to previously when treating of water 
injection. 

The clearance in the initial cylinder is rilled with air at a pressure 
less. than the receiver pressure; and as the diameter of the high pressure 
cylinder is small, the loss in capacity by clearance is reduced. Mr. E. Hill 
states that a single type of compressor should <have a stroke of 68 in., in 
order that its clearance loss sihall not exceed that of a compound com- 
pressor of 24 in. stroke when both compressors are forcing air against 60 
pounds pressure. 




CPfSCT OFC».t»*»k«Cft 



FIG. 15. 

Fig. 15 illustrates the effect of clearance in loss of volume in an air 
compressor. The diagram illustrates a typical case in compressors of 
cheap design and manufacture, the loss in volume amounting to about 
20 per cent. 

Unnecessary complications have been applied to air compressors to 
overcome clearance loss. Mr. Sturgeon has designed an air cylinder 
with lifting heads, so that the piston slightly raised the head at each 
stroke, thus reducing clearance to an exceedingly small figure. 

A simple method of reducing the clearance loss is by a passage or 
by-pass grooves in the end of the cylinder arranged in such a way that 
when the piston reaches the end of the stroke it passes over the grooves 
and thus allows the high pressure air, which is confined in the clearance 
space, to pass to the other side of the piston. The air in such cases is 
usually allowed to pass through the grooves at a velocity not exceeding 
too ft. per second. This plan is, however, subject to objections, and it is 
doubtful that there is anything gained by it. The load on the air piston 
is suddenly removed when the grooves are uncovered, and thus the 
cushion behind the piston is destroyed, and unless the cushion in the 
steam end is effective the compressor will pound badly at the end of each 
stroke. 

The commonest form of inlet valve applied to air compressors is the 
poppet. It is plain that as soon as the piston starts from the extreme point 
oi stroke nearest the cylinder head, it is followed by the air confined in 
the clearance space until that point is reached when the pressure in the 
cylinder is equal to the pressure outside. From this point the action of 



COMPRESSED AIR PRODUCTION. 35 

the piston is now to produce a partial vacuum within the cylinder, result- 
ing in opening the poppet inlet valves against the tension of the spring 
which holds them to their seats. The result of all this is that the poppet 
valve construction does not admit of a full air cylinder, as no air from the 
outside is admitted during a portion of the stroke. Even after the inlet 
valves are opened there is more or less friction through the passages, 
which reduces the atmospheric pressure. There is seldom less than one 
pound per square inch difference in pressure between the air within and 
without the cylinder. This, though apparently small, results in a very- 
serious reduction of efficiency. 

A common defect in air compressor construction is insufficient space 
for proper delivery after compression. The discharge valves and dis- 
charge passages are so contracted as to offer considerable resistance to 
the passage of the air, which should be discharged at the same velocity as 
that at which the piston moves. As the area in the cylinder head is 
usually small, the volume of valve area is restricted to the minimum. 
Heavy spring's are used in order to prevent the valves from hammering, 
and the result of this is a loss of power. It is not a difficult matter to 
admit the air through a valve which is moved by direct mechanical con- 
nection, and without springs, but many difficulties are in the way of posi- 
tive movement in discharge valves. The exact point when it is desired to 
admit the air is fixed for all pressures and temperatures, but not so with 
the point of discharge. This varies in proportion to the pressure, and 
this is effected more-w less by the conditions of temperature, dryness 
and density of inlet air, and the cooling effect during compression. It 
would not do to hold the discharge valves closed after the pressure in the 
cylinder has reached the point of pressure in the receiver. Equally 
serious in loss of efficiency would it be to open a discharge valve before 
the air in the cylinder has reached the receiver pressure; hence the poppet 
form of discharge valve is generally used. 

Designers of air compressors seldom consider the velocity at which 
the compressed air is discharged from the cylinder. When the discharge 
valves are first opened the piston is moving with considerable velocity, 
which discharges the air in some cases at a velocity of 200 ft. per second. 
This is, of course, gradually reduced as the piston nears the end of the 
stroke. 

Poppet valves when used either for inlet or discharge should be small 
in diameter and light in weight. Various, designs involving poppet 
valves of large diameter have been applied to compressors, but trjey have 
invariably failed because the increased inertia of a large valve will pound 
the seats and break the springs. Where poppet valves are used, a large 
number of small, light valves is the best construction. The movement 
of a poppet valve should be as short as possible, in order to minimize the 
wear which results from constantly striking the seat. 

It is important in designing an air compressor to provide ample inlet 
space, so that the cylinder will be filled with air and thus maintain the full 
volumetric capacity of the machine. The usual area of inlet is 1-10, the 
area of the piston, though in slow speed compressors this might be re- 



36 



COMPRESSED AIR PRODUCTION. 



duced considerably. Compressors provided with a concentrated inlet 
through which the air is drawn alternately at one end and the other of the 
piston, do not require as large an area of inlet as the intermittent type, 
because the air when once started through the inlet is maintained con- 
stantly moving in one direction and toward the cylinder; thus a draft of 
air is produced which materially aids in piling up the pressure at the point 
of reversal of stroke. It must be borne in mind that while the piston is 
drawing the air into the cylinder its speed is variable. At the beginning, 
when turning the centre, it is slow, increasing rapidly until it reaches its 
maximum speed at the centre of the stroke and then decreasing until it 
reaches the point when the inlet valve closes. At that point the piston is at a 
standstill, and inasmuch as its speed has been gradually reduced, it is nat- 
ural to expect that the velocity of the air when started at a high rate is to a 
certain degree maintained, resulting in a completely filled air cylinder. 





Tabi^e No. 5.- 


—Proportions 


of Air Cyundfrs. 








Per Ct. of 


Per Ct. of 


Area of 
Cylinder. 
Sq. inch. 




Air Inlet. 


Air Discharge. 


Sizes of Air 


Clear- 


Clearance. 












Cylinders. 


ance. 


Air Comp'd 
























Free Air. 


to 75 lbs. 


Size of 


Area of 


Per Ct. of 


No. of 


Area of 


Per Ct. 






.047 


~ 78~~ 


Pipe. 
2 in. 


Pipe. 

3-i4 


Area. 
.04 


Valves 
2 


Valves. 
5.4 sq. in. 


Of Area 


io}{ in. x 12 in. 


.0094 


.07 


12^ in. x 14 in. 


.0086 


•043 


113 


2% in. 


4-9 


•043 


2 


8.8 


.078 


14 # in. x 18 in. 


.0066 


•033 


154 


3111, 


7- 


•045 


3 


13.2 


.085 


16^ in. x 18 in. 


.0066 


•0330 


201 


3/^in- 


9.6 


.047 


3 


13.2 


.065 


i8}£ in. x 24 in. 


.0049 


.0225 


2.S5 


4 in. 


13- 


■Q5 1 


8 


35.2 sq. in. 


.14 


2o}i in. x 24 in. 


.0049 


.0225 


3H 


4% m. 


16. 


.031 


8 


35-2 


.11 


22^ in. x 24 in. 


.0049 


.0225 


380 


5tn 


20. 


•053 


10 


44. 


.116 


30% in. x 60 in. 


.002 


.01 


707 


6 in. 


28. 


.04 


18 


79.2 


.112 


36^ in. x 48 in. 


.002 


.01 


1018 


7 in. 


38.5 


.038 


20 


88.. 


.086 



The clearance on the ends of cylinders is 1-16 inch on each end, and is the same on all cylin- 
ders, for pressure up to 100 lbs. On high pressure cylinders the end clearance is reduced to 0. The 
percentage of clearance at 75 lbs. is taken as 5 times free air, thus allowing heating. 

Table No. 5 gives the proportions of air cylinders, valves and clear- 
ance spaces in air compressors of the. best modern design. In this case 
the inlet is concentrated; that is, the air is drawn into the cylinder through 
a piston inlet tube, hence five per cent, of the area of the cylinder is suffi- 
cient for the inlet. These dimensions have been practically tested, and 
in no case, even at the maximum speed of the machine, has there been any 
evidence of contraction. 

The discharge valve area depends upon the speed of the compressor, 
and for the best results should be about ten per cent, of the cylinder area 
for a piston speed of 300 feet per minute, or fifteen per cent, for a speed 
of 450 or 500 feet. 

In the usual air-compressor plant the free air is taken in from the 
engine room. This is a mistake which is so apparent that it is difficult 
to account for its existence. It is not only desirable to compress clean, 
dry air (which is not usually found in the engine room), but the economy 
and capacity of an air-compressing plant are largely dependent upon the 
temperature of the intake air. A low temperature increases the capacity, 
and adds to the efficiency of the machine. 



COMPRESSED AIR PRODUCTION. 3) 

A concentrated inlet connecting the valves with a point outside the 
engine room is the best construction. A chimney made of wood will 
ierve the purpose very well, though cooler air can be obtained by draw- 
ing it from a well. The presence of water in the well is no disadvantage, 
provided the water is cold. The intake air might advantageously be 
passed through cold water, or subjected to a spray of water for the pur- 
pose of reducing its temperature and destroying the impurities. 

For every five degrees that the temperature of the intake air is low- 
ered below that of the engine room, there is a gain of one per cent, in 
volume. In summer only a few degrees difference can be obtained but 
in winter the difference between the air inside and outside is often fifty de- 
grees. 

Mr. E. Hill has compiled the following table, showing the discharge 
of a compressor having an intake capacity of 1,000 cubic feet of free air 
per minute; the temperature of the intake air varying from zero to no 
degrees, and in each case being discharged at a uniform temperature of 
sixty-two degrees F., and at atmospheric pressure: 

TemperatureVolume taken Volume discharged, if 
of into measured at 62 degrees 

Intake Air. Compressor. and 

atmospheric pressure. 






J 000 


cubic ft. 


1135 


cubic ft. 


32 


1000 


" 


1060 


" 


62 


1000 


" 


1000 


" 


75 


1000 


" 


975 


" 


80- 


1000 




966 


" 


90 


1000 


'« 


9±9 


" 


100 


1000 


" 


932 


" 


no 


1000 


" 


916 


* l 



Mr. R. A. Parke, in a paper read before the New York Railroad 
Club, illustrated the great importance of supplying the compressor with 
cold air, as follows: 

"With an ordinary single-stage compressor, furnished with no cool- 
ing appliance, the delivery temperature of the air at the compressor is 
about 437 degrees. It may be assumed that, in every ordinary case of 
transmission, the temperature of the air falls to that of the locality where 
it is used, with an accompanying shrinkage of volume, so that, in the 
present example, the temperature of the air used would be 60 degrees, 
and each cubic foot of air delivered at the compressor shrinks to .58 cubic 
feet before being used. If the temperature of the atmospheric air re- 
ceived in the compressor were 40 degrees instead of 80 degrees, the de- 
livery temperature would be but 371 degrees, and, when cooled to 60 
degrees, the shrinkage of each cubic foot delivered would be only to .626 
cubic feet. Thus eight per cent, greater volume of atmospheric air, at a 
temperature of 80 degrees, must be compressed than would be necessary 
if the temperature of the air were 40 degrees." 

Low temperature of intake air is so important that the plants estab- 
lished for the purpose of compressing air for power distribution should 
be located with this question prominently in mind. An ice making 



3& COMPRESSED AIR PRODUCTION. 

plant, or a cold-storage room, is the best place for an air-compressor. In 
such a place as this, with the temperature of the intake air at or near zero, 
the compressor, provided with compound cylinders, jacketed by a circula- 
tion of cold brine, and thoroughly cooled between the stages, we might 
produce compressed air at a low temperature and a high efficiency. This 
cold, dry air, distributed over or under ground (thus increasing its tem- 
perature in transit), and heated before use, might develop an efficiency 
beyond that which is pqssible in the use of electricity, hydraulics, or any 
other form of secondary power. 

AIR COMPRESSING MACHINES. 

In describing air pumps or compressors we will endeavor to confine 
ourselves strictly to such machines as are at the present time in practical 
operation. The large field of patented designs, antiquated types and such 
like, will be omitted. 

The simplest form of air pump is the little apparatus with which 
every one is familiar, namely, the pump for inflating the bicycle tire, 
Fig. 1 6. 



FIG. 16— A BICYCLE AIR PUMP. 

This little pump is made both single and double acting, the single 
acting device being the simplest form of air compressor. The piston rod 
is the discharge passage, the piston packing being a simple leather 
washer which is aided by the air pressure in forming a perfectly tight 
joint, the piston is fixed, the cylinder being grasped in the hand and 
forced forward. This little device is interesting in that its use reminds 
us of an important fact in air compressing — the development of heat. 
After a few strokes the heat produced is quite perceptible. This heat is 
not due to friction, but it represents the conversion of the work done by 
the hand into heat. When there is little resistance or little air in the tire 
the heating is not noticed because but little work is* done. As soon, how- 
ever, as the tire becomes inflated the action of the pump is more difficult 
to perform and the heat increases. It is not an uncommon experience 
for this pump to fail to act, due to the sticking of the little check valve 
which connects the cylinder through the piston rod with the interior of 
the tire. When this valve is stuck it will be noticed that on pressing the 
pump cylinder forward and letting go, it is quickly pushed back to its 
original position, heat has been produced and there is evidently a pressure 
within the cylinder tending to hold its front end firmly against the 
piston. Where does this pressure come from? We had only free air 
and no pressure before giving the stroke to the cylinder and now it has 
returned to its original position and tends to go further. There has been 
no leakage from the tire because the check valve is tight and immovable. 



COMPRESSED AIR PRODUCTION. 



39 



We have simply compressed a volume of confined air as we would com- 
press a spring, yet unlike a spring it apparently has power to go further 
than its original position. This is because we have done work upon the 
air, it has failed to get away, heat has been oroduced because the air has 
done no work in return. This heat expands the air, tnus giving it addi- 
tional power to recover, and as the cylinder is a light thing it is pushed 
back to its original position with but little expenditure of energy. If the 
pump and its confined air are now allowed to cool down to the tempera- 







FIG. 17. 



ture of the surrounding atmosphere the tension in the cylinder will be 
relieved and the original conditions will be recovered. Even a bicycle 
pump may teach us important lessons in thermodynamics. 

Hand pumps of larger size are made for medical service and other 
uses, the principle being simply that of a piston forced backward and for- 
ward in a cylinder. Where larger volumes of air are required hand pumps 
provided with fly-wheels and operated by one or more men have been 



40 



Compressed air production. 



introduced and are largely used. The commonest pattern is the Vertical 
Belt or Hand Air Pump (Fig. 17) which may be run by hand or by power 
as desired. 

The High-Pressure, Vertical, Belt Air Pumps are built in response 
to many applications for a reliable and small belt or hand power air pump 
to compress air to 300 pounds pressure. There have been several more 
or less important improvements made on the original design, and on the 
larger sizes the suction and discharge pipes are cast solid to the cylinder. 

They can be bolted against any pillar, wall or strong board, and 
driven by a heavy pulley wheel of such diameter and width of face as re- 
quired to attain the pressure wanted; when required to be worked by hand 
power they are provided with a handle in fly-wheel, which can be run by 
one or two men to a proportionate pressure. 

The air brake pump is the simplest form of power air compressor 
with which we are familiar. Furthermore, it is a machine so largely used 
that it deserves more than a passing notice. There are more air com- 



VERTICAI,, 



BEI/f OR HAND AIR PUMPS. 
PRICE UST. 





"^ 


a 


h 


% -• 


1/3 n" 
4> W 




u 


u . 

en 

3-S 


M 



. 
> <u 






V 

.0 


«« fl 


ai 


<« S 




<J« 


V 


E 


°« 


HH 


°.E 


^ qj-O 


ll' - ' 


S-c 


3 

5? 




O . 

en 


u ft 


2.2 


P4 


"S V 


t 3 ^ 


3-S 




g 0^3 










p'o ft 


Jj tO.,1 






s 


►4 


£ 


a 


<3 




1 


2 


6 


150 


1 


300 


$50 


2 


2% 


6 


150 


2 


300 


50 


3 


3 


6 


150 


3 


250 


60 


4 


4 


6 


140 


6 


200 


70 


5 


5 


6 


140 


10 


150 


80 


6 


6 


6 


130 


13 


100 


90 


7 


7 


6 


130 


17 


50 


110 


8 


8 


6 


120 


20 


30 


130 



pressors used for the purpose of operating brakes than for all other pur- 
poses combined. It has been estimated that 30,000 of these pumps are 
in service. Fig. 18 illustrates the improved 9J inch Westinghouse pump. 

THE NINE AND ONE-HALF INCH IMPROVED AIR PUMP. 
DESCRIPTION OF THE AIR BRAKE PUMP. 



60. Top Head, complete (Includes parts Nos. J2, J$, 74, 75, j6, 83, 84, 

85, 108, 109, and 8 of no. 

61. Steam cylinder, complete. (Includes parts Nos. 90, 91, 92, 93 and 

94-) 



COMPRESSED AIR PRODUCTION. 




Win""' 



n».i 



Al^o LJJ 



Fto-2 



FIG. 18. 



62. Centre Piece, complete. (Includes parts Nos. 2 each of gs, 06 and 
97.) y:? ' y 

63. Air cylinder, complete. (Includes parts Nos. 4 of 86, 2 each of 87 

88 and 89, and 1 each of 90, 91, 92, and 106.) 



42 Compressed air production, 

64. Lower Head. 

65. Steam Piston and Rod. (Includes 2 each of 67 and 68, 69 and 2 of 
70.) 

66. Air Piston, complete. (Includes 2 of 67.) 
6y. Piston Packing Ring. 

68. Piston Rod Nut. 

69. Reversing Valve Plate. 

70. Reversing Valve Plate Bolt. 

71. Reversing Valve Rod. 
J2. Reversing Valve. 

73. Reversing Valve Chamber Bush. 

74. Reversing Valve Chamber Cap. 

75. Main Valve Bush. 

76. Main Piston Valve, complete. (Includes Nos. yy, 2 of 78, 79, 2 of 
80, 81 and 4 of 82.) 

yj. Large Main Valve Piston Head. (Includes 2 of No. 78.) 

78. Large Main Valve Piston Packing Ring. 

79. Small Main Valve Piston Head. (Includes 2 of No. 80.) 

80. Small Main Valve Piston Packing Ring. 

81. Main Valve Stem. 

82. Main Valve Stem Nut. 

83. Main Slide Valve. 

84. Right Main Valve Cylinder Head. 

85. Left Main Valve Cylinder Head. 

86. Air Valve. 

87. Air Valve Seat. ■ 

88. Air Valve Cage. 

89. Valve Chamber Cap. 

90. One and One-fourth Inch Union Stud. 

91. One and One-fourth Inch Union Nut. 

92. One and One-fourth Inch Union Swivel. 

93. One Inch Steam Pipe Stud. 

94. Governor Union Nut. 

95. Stuffing Box. 

96. Stuffing Box Nut. 

97. Stuffing Box Gland. 

98. Air Cylinder Oil Cup. ; J 

99. Short Cap Screw. 
ico. Long Cap Screw. 

101. Upper Steam Cylinder Gasket. 

102. Lower Steam Cylinder Gasket. 

103. Upper Air Cylinder Gasket. 

104. Lower Air Cylinder Gasket. 

105. Drain Cock. 

106. Air Strainer. 



COMPRESSED AIR PRODUCTION. 43 

toy. One-Inch Steam Pipe Sleeve. 

108. Left Main Valve Head Gasket. 

109. Right Main Valve Head Gasket, 
no. Main Valve Head Bolt. 

This air pump was designed without any thought of economy in the 
use of steam, the main consideration being economy of space, light weight 
and absolute reliability of action. Economy in the use of steam to compress 
the air is quite secondary to other and more important considerations. 
The use of a crank and fly-wheel is incompatible with economy of space, 
and ia compressor with directly operated steam valves is a necessity. Be- 
cause of this the clearance space in the air cylinder must be about double 
that required in a fly-wheel machine in order to avoid pounding. The 
steam ports, and especially the exhaust ports, must be contracted to pre- 
vent too high piston speed and consequent pounding when the pump is 
working against low air pressures. For the same reason the air discharge 
valves must also be small. The steam must be used non-expansively, as 
the resistance of the air pressure is greatest toward the end of the stroke, 
requiring the highest steam pressure at that time, and having no inertia 
of moving parts to help it around the center. 

Several years ago experiments were made by the Westinghouse Com- 
pany, extending over a period oif two years, the purpose being to deter- 
mine the question of steam economy. The eight-inch pump had been de- 
signed at a time when the average locomotive boiler pressure was about 
120 lbs., and in order to secure an abundant supply of air the area of the 
steam piston was made about 15 per cent, greater than that of the air 
piston. Between that time and the time when the tests were begun, the 
steam pressure of locomotive boilers had become considerably increased, 
so that the greater >area of the steam piston of the air-pump was no longer 
necessary. Experiments were conducted, both with a simple pump of 
suitable proportions to meet the changed conditions of service and with a 
compound pump, designed to attain the highest steam economy subject 
to the peculiar limitations of the service. 

The following descriptions of these tests are given by Mr. Parke in 
his paper before the New York Railroad Club: 

"The design of compound pump which seemed to best meet the re- 
quirements has two cylinders, respectively six inches and ten inches in 
diameter, and each of 10 inches stroke. Live steam is admitted to the 
smaller or high-pressure steam cylinder throughout the entire stroke, and, 
upon the return stroke, it expands into' the larger or low-pressure steam 
cylinder, no live steam being admitted to the latter. All the ports of both 
steam cylinders are controlled by a single steam valve. There are two 
air cylinders, the diameters of which are respectively 6\ inches and 9J 
inches, and the stroke of each is 10 inches. Atmospheric air is drawn into 
the larger or low-pressure air cylinder, and compressed therefrom into the 
smaller or high-pressure air cylinder. In the latter, the air is further 
compressed and delivered thence to the main reservoir. The piston of 
the high-pressure steam cylinder operates that of the low-pressure air 



44 



COMPRESSED AIR PRODUCTION. 



cylinder, and the piston of the low-pressure steam cylinder operates that 
of the high-pressure air cylinder. The reversing valve is actuated by the 
high-pressure steam piston. By this arrangement, the complete stroke 
of both the high-pressure steam piston and the low-pressure air piston is 
always assured, so that the pump cannot become dead, and a cylinder full 
of free air is always secured. The pump is operative at any steam pres- 
sure, the pressure at which the air can be delivered depending of course 
upon the available steam pressure. 

The design of simple pump resulting from these experiments was 
what is now known as the O/J-inch pump, which has one steam and one 
air cylinder, each 9 \ inches in diameter, with a 10-inch stroke. 

In the tests of the various pumps, as to efficiency and capacity, the 
steam was condensed in a surface condenser, at atmospheric pressure and 
weighed, and the volume of air actually delivered was carefully measured. 
The conditions under which the tests were made were those of about the 
average service to be expected on the road; this is, the steam pressure 
used was 140 pounds and the pumps were required to deliver against an 
air pressure of 90 pounds. 

Table VI. indicates the capacities and efficiencies of the various types 
of air-brake pumps under these conditions. For comparison, this table 
also indicates the volume of free air compressed to 90 pounds and de- 





Volume of Free Air 

Compressed to 90 lbs., 

and Discharged. 


Pump. 


Per Min. 


Per Pound 
of Steam 
at 140 lbs. 
Pressure. 


8-inch 

9^-inch 

Compound 

5-inch Duplex 

7 « 

Simple Compressor, oper- 
ated by simple engine . 

Two-stage compiessor 
operated by compound 
non-condensing engine 


36.3 cu. ft. 
44.9 
43 3 
39.4 

38.7 


1.85 cu. ft. 

3.49 
4.89 
3.06 
3.43 

8.80 
13.70 







livered per pound of steam by a single stage commercial compressor, 
operated by an efficient simple engine, and by a two-stage compressor, 
with intercooler, operated by a compound non-condensing engine. 

The most striking feature of this table is the low efficiency of an air- 
brake pump, in comparison with a suitable compressor for a commercial 
supply of compressed air. Steam generation in stationary boilers for 
ordinary power purposes, is comparatively uniform, and the amount of 
fuel burned is practically proportional to the quantity of steam used. 
Ur>der such conditions, unless the volume of compressed air required is 



COMPRESSED AIR PRODUCTION. 45 

very small, or is required at irregular intervals and in uncertain quan- 
tities, it is not economy to use an air-brake pump in the place of a suitable 
compressor. 

At the conclusion of these pump tests, it was decided to place the 
Qj-inch pump upon the market, and, although the design of compound 
pump selected proves entirely satisfactory as to capacity, and requires 
only one-half the steam used by any air-brake pump in the market, it 
was decided to abandon any thought of offering it for sale. The reasons 
for the latter decision w T ere chiefly the following. 

While the only working parts added to those of the simple pump are 
the additional pair of pistons and two additional air valves, a long ex- 
perience has led to the conviction that simplification and not complica- 
tion of air pump construction is what the best interests of the railroads 
require. The increased number of parts necessarily implies greater cost 
of maintenance and renewals, additional glands to be kept packed, a con- 
siderably increased number of sources of leakage, and the additional pair 
of cylinders materially increses the bulk and the weight of the pump. 
This was regarded as the most serious objection to the introduction of a 
double pump. 

Another serious objection is heating of the air end of the pump. It 
has been fully explained that compounding, or compressing in stages, is 
the method most to be preferred, when the air is cooled between the 
stages. It might also have been stated that for practical reasons, com- 
pounding the air, without cooling between the stages, is the worst method. 
By this method the air is theoretically delivered by the high-pressure 
air-cylinder at about the same temperature as it is from the air cylinder 
of a simple compressor; but the temperature of the air taken into the 
cylinder of a simple compressor is that of the atmosphere, while that of 
the air taken into the high-pressure air cylinder of the compound method 
(without cooling) is from 200 to 300 degrees above that of the atmos- 
phere. It is evident, therefore, that the mean temperature of 
the air in the high-pressure air cylinder of the compound method 
is very much higher than that of the air in a simple com- 
pressor. Indeed, it several times occurred, during the experiments 
referred to, that when a pump of the two air-cylinder type was allowed 
to run freely, for twenty-five or thirty minutes, under the conditions stated, 
the maple plank upon the brick wall of the testing room to which the air 
cylinders were bolted, took fire. Such high temperatures of the high- 
pressure air cylinder are productive of serious evils. The two air cylin- 
ders are necessarily cast in one piece, of an irregular shape, and expansion 
by heat is inevitably accompanied by distortion. This distortion at the 
air cylinders is aggravated by the still further increased temperature 
resulting from binding of the pistons. Unless the air pistons are 
originally fitted so loosely as to permit them to leak badly, they bind and 
cut, causing great wear and materially increasing the cost of maintenance. 
The distortion of the air cylinders always causes a large amount of leak- 
age past the pistons, and the actual efficiency of such pumps is far below 



46 



COMPRESSED AIR PRODUCTION. 



that calculated for them. As it is wholly impracticable to introduce a 
cooling chamber upon a 'locomotive, in connection with the compound 
air cylinders, this type of pump seems to be very undesirable. 

The one other important reason for abandoning- the compound pump 
is that steam economy in an air-brake pump is not of importance or value. 
Such a conclusion may at first cause some surprise, but a little study of 
the conditions will fully support it. The steam requirements of a locomo- 
tive are very difficult ones to meet, as the demand for steam to operate 
the engine fluctuates very greatly. The steam required by an air-brake 
pump is not only an exceedingly small proportion of the quantity gen- 
erated, but it also fluctuates between limits widely apart. The air- 
brake pump does its heaviest duty while stading at stations and in 
descending grades. 

At such times the engines of the locomotive use no steam, and such 
steam as is then used by the air-brake pump would otherwise probably 
escape through the pop valve. It is well known that, with the most care- 
ful firing, it is practically impossible to prevent a waste of steam through 
the safety valve at such times. 




FIG. 19.— DIRECT ACTING STEAM AIR COMPRESSOR. 

In connection with a series of tests of a Baldwin compound locomo- 
tive upon the Baltimore & Ohio R. R., in 1890, the consumption of steam 
in various incidental ways was ascertained. The weights of steam which 
passed through the whistle, the blower and the pop valve were computed. 
Instead of allowing the pop valve to blow off, a vent pipe from the steam 
dome was carried into the tender, and the weight of steam which would 
pass through it per second, when a valve in the pipe was opened, was 
ascertained. During the runs, when the steam reached such a pressure 



COMPRESSED AIR PRODUCTION. 47 

that the pop valve was on the point of blowing off, the valve in the vent 
pipe was opened, and, by noting the length of time that it remained open, 
the quantity of steam which so escaped was easily computed. The num- 
ber and lengths of blasts of the whistle were also noted, together with the 
time that the blower was used. The amount of steam used by the 8-inch 
air-brake pump was also computed. It was found that, after the train 
brake apparatus had become fully charged, the pump continued to run 
with an average speed of about 62 strokes per minute. This would seem 
to indicate a considerable amount of leakage in the train pipe and con- 
nections. The writer has observed the operation of the 8-inch pump upon 
a considerable number of different trains, the air brake apparatus of which 
was in such condition as he happened to find it. It was found that, after 
the brakes had been charged throughout the train, the number of strokes 
per minute made by the pump were from 24, for a four-car train, to 47, 
for a seven-car train, the average of all the cases observed being 36. 

During the trials of the Baldwin compound, three separate runs were 
made from Baltimore to Philadelphia, with the same train, making the 
same time, and with the same number of scheduled stops. In the report 
of Mr. George H. Barrus, the steam used in different ways during these 
runs was computed as follows: 

Safety 
Whistle. Blower. Valve. 

415 lbs 153 lbs 220 lbs. 

503 lbs 198 lbs 99 lbs. 

425 lbs 72 lbs 302 lbs. 

Average, 448 lbs 141 lbs 207 lbs. 

The average computed steam consumption of the air pump, under 
the conditions of its actual operation, was 690 lbs. Under the ordinary 
conditions of the brake apparatus in service, with the air pump making 
about 36 strokes per minute to keep up the pressure after the apparatus 
is once charged, the steam consumption, computed upon the same basis 
as that used in the report of these locomotive trials, would have been 
400 lbs. It will be seen from these figures, therefore, that about the same 
quantity of steam w^as used by the whistle as is used by the air 
pump, and that, even in these locomotive trials, w T here unusual care was 
undoubtedly exercised in firing, the loss of steam by the safety valve was 
about 'half that which should be used by the air pump. 

Under the ordinary conditions of service, although the steam used 
by the air pump is largely that which would otherwise be lost at the 
safety valve, it is most probable that the amount of steam lost at the 
safety valve is greater than that used by the air pump. 

It will thus be understood that, with the fluctuating conditions of 
locomotive service, the steam consumption of the air-brake pump is an 
insignificant factor in the economy of steam production. It has never 
been demonstrated, so far as the writer is aware, that a locomotive haul- 
ing an air-braked train, with the brake apparatus in ordinarily good con- 



48 



COMPRESSED AIR PRODUCTION. 



dition, consumes, in the long run, a pound more coal than the same 
locomotive, without an air pump, would use in hauling the same train 
when braked by hand, and there is no good reason to believe that it 
would. 

Fig. 19 illustrates the simplest type of small air compressor built on 
the Straight Line or Direct Acting plan. The machine is designed for 
light transportation and for use in places where a compact, self-contained 
and very simple compressor is wanted. The steam and air cylinders are 
connected to a crosshead which moves in guides arranged between the 
cylinders. The fly wheels, which are joined by connecting rods to this 
crosshead, are mounted in bearings close to the steam cylinder. Slide 
valves are used operated by eccentrics, though in some of the larger 




FIG. 20. 



sizes the adjustable cut-off serves to aid this type of machine in fuel 
economy. The air cylinder is provided with poppet, inlet and discharge 
valves, and is water jacketed. Simplicity of design is followed in 
machines of this type, no claims being made to high fuel economy. 

Fig. 20 shows another type of simple, low cost, single or direct act- 
ing compressor. No crosshead is used. A single fly wheel with crank 
shaft and connecting rod are placed between the cylinders. 

Fig. 21 is shown for the purpose of illustrating the English type of 
single or straight line compressor. The machine shown is known as 
"Schram's patent." A single cast iron bed plate is used, thus making the 
machine self-contained, and the fly wheel's crosshead and connecting rod 



COMPRESSED AIR PRODUCTION. 



49 




FIO. 21. 



are placed on the bed, the steam cylinder being located between these 
parts and the air cylinder. This construction brings the steam and air 
cylinders close together, but the machine is not as compact or as simple 
as the direct acting compressor of the American type. 

Another form of European compressor on the straight line plan is 
shown in Fig. 22. This machine is made by Messrs. Burkhardt & Co., 




FIG. 22. 



5<> 



COMPRESSED AIR PRODUCTION. 



Basel, Switzerland. The steam and air cylinders are widely separated 
and are connected by a single piston rod joined at or near its centre by a 
crosshead, to. whidh 'the connecting rod is attached. 

Fig. 23 illustrates a type of straight line compressor of the larger 
sizes. The following" are the specifications on which these mac'hmes are 
built: 

Cylinders. — Steam Cylinder, 14 inches in diameter, by 18 inch stroke. 
Air Cylinder, 14 J inches in diameter, by 18 inch stroke. 
Cylinders made of the best cast iron suitable for this purpose, and of 
proper strength and thickness for carrying 100 pounds pressure, after 
being re-bored once. 

Water Jacket. — Air Cylinder and heads provided with Water Jackets. 




FIG. 23. 



Bed Plate. — Bed Plate of the Box Girder type, made in a single cast- 
ing. The bed plate to extend throughout the compressor connecting both 
cylinders and all parts, and strong enough to withstand the severest strain 
of air compressing work. 

Bearings. — Bearings to form an integral part of the bed plate, pro- 
vided with removable bronze boxes, adjustable, accurately bored, and 
scraped to a true bearing surface. 

Shaft. — Main Shaft of hammered steel, 6 inches diameter in the bear- 
ings, turned, finished and key-seated, and free from flaws or other imper- 
fections. 

Fly Wheels. — Two square rim Fly Wheels, 6 ft. o in. in diameter, 
with face and edges nicely turned and hubs bored >and key-seated to fit 
shaft. The wheels to weigh when finished not less than 3,125 pounds. 



COMPRESSED AIR PRODUCTION. 5 I 

Crank. — Crank pins of best hammered steel and securely fastened in 
fly wheels, and fly wheels counterbalanced. 

Valve Gear. — Valve Gear of the slide valve type, with Meyer adjust- 
able cut-off gear. 

Piston Rod. — Piston Rods to extend through back head of steam 
cylinder and front head of air cylinder, and securely fastened in cross- 
head. 

Cross-head. — Cross-head of cast steel and amply strong, provided 
with bronze shoes, and with adjustment for piston rods. 

Material Used. — Piston Rods, Valve Rods, Connecting Rods and 
Crank Pins, of the best forged steel. Boxes for crank and cross-head 
pins of composition metal. 

Throttle Valve. — Steam Cylinder provided with a Globe Throttle 
Valve, with flanges and hand wheel turned and polished. Valve provided 
with drain connection. 

Governor. — The Compressor is provided with an Automatic Pressure 
Regulator and unloading device. 

Indicator Connection. — Provision made on both steam and air cylin- 
ders for Indicator connections. 

Oiling Devices. — Oil Cups for all wearing parts ; one sight feed Lu- 
bricator for steam cylinder, one Ingersoll-Sergeant Lubricator for air 
cylinder. All oiling devices extra large. 

Weight. — Total weight of Compressor ready for shipment, 10,800 
pounds. 

Where larger volumes of air are required the direct acting type gives 
place to the compound and duplex. It is not considered good practice to 
build single compressors with air cylinders larger than 26 inches in di- 
ameter, except where the air is compounded. 

Fig. 24 illustrates a popular type of Direct Acting Compound Com- 
pressor. The merits of this compressor are referred to by the manufactur- 
ers as follows: 

"The large air cylinder on the left determines the capacity of the 
Compressor, and for the illustration we have taken its piston at 100 square 
inches area. The small air cylinder in the centre can have an area of 33 1-3 
square inches. The small piston only encounters the heaviest pressure, 
and atioo lbs. pressure the resistance to its advance is 3,333 lbs. The re- 
sistance against tne large piston is its area multiplied by the pressure 
which is caused by forcing the air from the large cylinder into the smaller 
cylinder, which is in this case 30 lbs. per square inch. But as this 30 lbs. 
pressure acts on the back of the small piston and hence assists the ma- 
chine, the net resistance of forcing the air from the large into the small 
cylinder is equal to the difference of the area of the two pistons multiplied 
by the 30 lbs. pressure. This is 66 2-3 by 30, and equals 1,999 l° s - Hence 
1,999 lbs., the resistance to forcing the air from the large into the smaller 
cylinder plus 3,333 lbs., the resistance in the smaller cylinder to compress- 
ing it to 100 lbs., is the sum of all the resistances in the compound cyl- 
inders at the time of greatest effort. This is 5,333 lbs. The time of great- 



52 



COMPRESSED AIR PRODUCTION. 

it 




Fig. 24 — Compound Air Compressor. 

Arrows on the water pipes show the direction of water circulation. 
When pistons move as indicated by the arrow on the piston rod, steam 
and air circulate in direction shown by arrows in the cylinders. 

O — Air Relief Valve, to effect 



A- 

B- 



-Inlet Conduit for Cold Air. 

■Removable Hoods oif Wood, easy starting after stopping with all 



C— Inlet Valve. 
i D — Intake Cylinder. 

E — Discharge Valve. 

F — Intercooler. 

G — 'Compressing Cylinder. 
, H — Discharge Air Pipe. 

J — Steam Cylinder. 

K — Steam Pipe. 

L — Exhaust Steam Pipe. 

'N — Swivel Connection for Cross- 
head. 



pressure on the pipes. 

1 — Cold Water Pipe to Cooling 
Jacket. 

2 & 3 — Water Pipe. 

4 — Water Overflow or Discharge. 

5 — Stone on end of Foundation. 

6 — Foundation. 

7 — Space to get at underside of 
Cylinder. . 

8 — Floor line. 



est effort is at the end of the stroke, or when the engine is passing the 
centre. In the single machine this resistance is 10,000 lbs., hence we see 
that in the compound machine the maximum strains are less by over 46 
per cent., or nearly one-half. By thus reducing the work to be done at the 
end of the stroke, more work is done in the first part of the stroke, and 
the resistance is made nearly uniform for the whole stroke. 

"The next step is to> render the application of power also uniform 
for the Whole stroke. This is accomplished in a very simple and effective 



Compressed air production. 53 

hianner. The steam and air pistons and crosshead are mounted on the 
same piston rod. These parts are purposely given weight enough so 
so that it requires most of the power of the steam over and above 
the air resistance at the beginning of the stroke to start them 
forward at the required speed. At the end of the stroke, 
when the steam has become weak by expansion, the power 
stored up in the momentum of these reciprocating parts is given out in 
useful work, and the parts are brought to a state of rest by expending 
their force upon the air in the compressing cylinders. As the energy 
which can thus be stored and given out by the reciprocating parts depends 
upon their weight, and the square of the number of revolutions, it is evi- 
dent that rotative speed is the most important factor. Hence very long 
strokes are not desirable, because at the same piston speed the machines 
make fewer revolutions than machines of shorter strokes. Therefore the 
power is not applied to< the work so uniformly, and greater strains are 
brcught on shafts, connecting rods and other parts, and larger fly wheels, 
and frequently double engines, are necessary for successful operation, 
especially when steam is to be used expansively. The value of rotative 
speed for economical steam consumption is too well known to need re- 
viewing here. It is of interest, however, to note that the quick rotation 
that is valuable for applying the power uniformly also contributes to 
steam economy. 

"Uniform resistance and uniform power both applied, as in this com- 
pressor, as direct end thrust and pull upon a straight steel piston rod, do 
not leave much work for fly-wheels to perform. Their presence, however, 
is necessary to regulate the steam valve motions, to control the length of 
stroke, to even up and balance trifling inequalities of power and resist- 
ance and to secure a uniform speed to the machine." 

Figure 25 illustrates a common type of Duplex Compressor where 
the steam end is of simple construction, provided with the Meyer valve 
gear, the cut-off being adjusted by a hand wheel while the machine is in 
motion. The point of cut-off is indicated iby a pointer which moves over 
a graduated scale. The machine is run with a wide open throttle, being 
controlled entirely by the cut-off, which is proportioned in accordance 
w r ith the steam and air pressures. An ordinary ball governor is used to 
regulate the speed of the engine and to prevent it from running away in 
case of breakage of air pipes or sudden loss of pressure. Attached to the 
ball governor manufacturers usually add a pressure governor or regulator 
which is used to reduce the speed when the air pressure reaches the max- 
imum. Various types of frames are used, some of them of the Corliss 
pattern ; but for the purpose of insuring stability, freedom from breakage, 
and to resist the sudden strains which are brought to bear during com- 
pression, the frames, bearings and fly wheels are usually heavy. The steam 
and air pistons are in some patterns tied together by a heavy cast iron 
sole plate and tie rod. The bearings are usually of phosphor bronze, are 
unusually broad, and are provided with means for taking up wear. The 
cranks are usually made of wrought iron, and the crank pins, crosshead 



54 



COMPRESSED AIR PRODUCTION. 



phi, piston rods, shafts, valve rods, links and pins, and all wearing parts, 
are of steel, and when possible this is hardened and ground. The heavy 
fly wheel gives a smooth, uniform motion, and aids in steam economy by 
admitting of early cut-off. When running one side of the compressor at 
a time, the fly wheel prevents irregularity of motion. In this type of com- 
pressor the air cylinders are usually fitted with poppet valves, although 




FIG. 25. 



manufacturers have succeeded in so far perfecting the positive moved 
valve and the piston inlet that they are applied with economy to the Meyer 
Duplex machines. The air cylinder is sometimes made Of hard brass, 
owing to the better conductivity of this material, and is as thin as can be 
made with safety. The cylinders of some machines are provided with 
jackets for water circulation, and the piston and piston rods are hollow. 



COMPRESSED AIR PRODUCTION. 



55 



A telescopic water tube is introduced at the back end of the cylinder, and 
cold water is supplied to aid in keeping - down the temperature during 
compression. The introduction of cold water in this way undoubtedly 
reduces the heat of compression, but it is subject to the many disad- 
vantages found in water injection. The water, unless free from foreign 
matter, is liable to destroy the cylinder, and even when pure it is dif- 
ficult to lubricate the parts, the water itself being a bad lubricant. This 




FIG. 26. 



applies to that type where the water passes through the piston and in 
contact with the walls of the cylinder between the rings and completely 
around the piston. It is obvious that this leaves a thin coating of mois- 
ture in contact with compressed air which is at a temperature much higher 
than that of the water, and as the volume of air is 'so much greater in 
proportion than that of the water, the result is an absorption of the water, 
which goes off into the air receiver in the form of moisture, to be afterward 
deposited in the pipes when the the temperature of the compressed air is re- 
duced in transmission. It is claimed by the makers that one of their Du- 



$6 COMPRESSED AIR PRODUCT I6ti. 

plex Meyer Cut-off Compressors was used and tested at Shaft No. 13 on 
the New York Aqueduct by Prof. James E. Denton of Stevens Institute 
of Technology, and that it produced a horse power with a consumption 
of. 25 pounds of steam per horse power per hour. > - J > ! . 

A duplex compressor can be regulated so as to run 1 at high or low 
speeds. It may even stop and start automatically. Where the air is used 
intermittently — that is, Where the use is irregular, — a governor is fur- 
nished which will adjust the machine to a slow speed when little air is 
being used, stop when no air is required, and start again when necessary, 
thus using the steam only in proportion as the work is done. 

The following are specifications of a type of Duplex Meyer Cut-off 
Valve Compressor: 

Cylinders — 2 Steam Cylinders, 14 inches diameter, by 18 in. stroke. 

2 Air cylinders, 14J inches? diameter, by 18 in. stroke. 

Cylinders made of the best cast iron suitable for this purpose, and of 
sufficient thickness to safely carry 100 pounds pressure after re-boring 
once. 

Piston Inlet. — Air cylinders of the Piston Inlet type. 

Water Jacket. — Air cylinders and heads provided with water jackets. 

Frame. — Frames of the Corliss Girder type, and strong enough to 
withstand the severest strain of air; compressing work. 

Bearings — Main pillow blocks provided with removable shell boxes 
of best composition metal, accurately bored and scraped to a true bearing 
surface. ' 

Shaft. — Main shaft of hammered steel, 6 inches in diameter in the 
bearings, and 7 inches in the centre, of proper length, turned finished 
and key-seated, and free from flaws or other imperfections. 

Cranks. — Cranks of the disc pattern, of selected charcoal iron, and 
of ample strength and proportion for the work required. 

Fly Wheel. — One square rim fly wheel, 8 feet in diameter, with hub 
bored and key-seated to fit shaft. The wheel when finished to weigh not 
less than 4,500 pounds. 

Valve Gear. — Valve gear of the slide valve type, with Meyer ad- 
justable cut-off. 

Piston Rods. — Piston rods extended through back head of steam 
cylinders, and attached by. means of couplings to piston rods of air 
cylinders. 

Tie Rods. — Air cylinders securely fastened to steam cylinders with 
tie rods. 

Governor. — A fly ball governor of approved pattern placed in main 
steam pipe, and driven with belt from main shaft. 

Lagging. — Steam cylinders covered with polished black walnut. 
Space between lagging and cylinder to be filled with mineral wool. 

Material Used. — Piston Rods, Eccentric Rods, Connecting Rods, 
Crank Pins, Cross-head Pins and Valve Rods of the best forged steel. 
Boxes for main crank pins of best /composition metal. 



COMPRESSED AIR PRODUCTION. 



$1 



Throttle Valve. — Both steam cylinders provided with a throttle 
valve, with flanges and hand wheel turned and polished. 

Indicator Connection. — Provision made on both steam and air cyl- 
inders for indicator connections. 

Oiling- Devices. — Graduating sight feed oil cups for all wearing 
parts; one sight feed lubricator for each steam cylinder; one Ingersoll- 
Sergeant Lubricator for each air cylinder. 

Weight. — Total weight of compressor ready for shipment, 22,500 
pounds. 




FIG. 27. 



Figure 26 represents a type of Vertical Compressor which consists 
of a steam engine connected by means of a crank shaft wrfh two single- 
acting air pumps, all placed in an upright position. It is compactly 
built, and is as economical as this type of machine will admit. The 
cranks are so placed in relation to each other that the greatest power 
of the engine is applied at the time of greatest resistance in the air 
cylinders. Water injection is usually applied to these machines, and it 
is a matter of note that it is in this compressor that water injection has 
had its longest service, and has given its best results. One reason for 



5^ COMPRESSED AIR PRODUCTION, 

this may be found in the fact that the cylinders being- placed vertically, 
are not subject to the wear and destruction which accompanies water 
injection machines of the horizontal type. This compressor has been 
largely used in the western part of the United States for mining serv- 
ice, though of recent years notably at the Anaconda, Copper Mines, 
it has been replaced by machines of a more improved and more econom- 
ical type on the pattern of the Duplex Corliss Compound. 

Figure 27 illustrates a type of vertical air compressor used in Eu- 
rope, known as the "Champion." Both the inlet and the outlet valves 
with their seats are arranged in the cylinder covers; the form of the 
frame is such that the valves can be readily moved without disturbing 
the cylinder cover joints. The inlet and outlet valves are provided with 
very long guides to insure their continued working without damage, and 
being placed vertically, the wear is more evenly distributed. The air 
cylinder and outlet valve boxes are surrounded by cold water jackets. 
The general construction of the machine is so clearly shown in the il- 
lustration that further description is unnecessary. 

The experience of American manufacturers, which has been more 
extensive than that of others, has proved the value of direct compression 
as distinguished from indirect, as shown clearly in this type of machine. 
By direct compression is meant the application of power to resistance 
through a single straight rod. The steam and air cylinders are placed 
tandem. Such machines naturally show a low friction loss, because of 
the direct application of the power to the resistance. This friction loss 
has been recorded as low as 5 per cent., while the best practice is about 
10 per cent, with the type which conveys the power through the angle 
of a crank shaft to a cylinder connected to the shaft through an addi- 
tional rod. 



9681 1^ if)i 



A 



